diff --git array.c array.c
index 3bbf935..e49732d 100644
--- array.c
+++ array.c
@@ -28,7 +28,7 @@
 
 VALUE rb_cArray;
 
-static ID id_cmp, id_div, id_power;
+static ID id_cmp, id_div, id_power, id_sort_by, id_to_ary;
 
 #define ARY_DEFAULT_SIZE 16
 #define ARY_MAX_SIZE (LONG_MAX / (int)sizeof(VALUE))
@@ -2501,7 +2501,7 @@ rb_ary_sort_by_bang(VALUE ary)
 
     RETURN_SIZED_ENUMERATOR(ary, 0, 0, rb_ary_length);
     rb_ary_modify(ary);
-    sorted = rb_block_call(ary, rb_intern("sort_by"), 0, 0, sort_by_i, 0);
+    sorted = rb_block_call(ary, id_sort_by, 0, 0, sort_by_i, 0);
     rb_ary_replace(ary, sorted);
     return ary;
 }
@@ -3571,7 +3571,7 @@ rb_ary_equal(VALUE ary1, VALUE ary2)
 {
     if (ary1 == ary2) return Qtrue;
     if (!RB_TYPE_P(ary2, T_ARRAY)) {
-	if (!rb_respond_to(ary2, rb_intern("to_ary"))) {
+	if (!rb_respond_to(ary2, id_to_ary)) {
 	    return Qfalse;
 	}
 	return rb_equal(ary2, ary1);
@@ -5532,8 +5532,10 @@ Init_Array(void)
     rb_define_method(rb_cArray, "drop_while", rb_ary_drop_while, 0);
     rb_define_method(rb_cArray, "bsearch", rb_ary_bsearch, 0);
 
-    id_cmp = rb_intern("<=>");
-    sym_random = ID2SYM(rb_intern("random"));
-    id_div = rb_intern("div");
-    id_power = rb_intern("**");
+    id_cmp      = rb_intern("<=>");
+    sym_random  = ID2SYM(rb_intern("random"));
+    id_div      = rb_intern("div");
+    id_power    = rb_intern("**");
+    id_sort_by  = rb_intern("sort_by");
+    id_to_ary   = rb_intern("to_ary");
 }
diff --git bignum.c bignum.c
index 6663057..ed11dba 100644
--- bignum.c
+++ bignum.c
@@ -13,6 +13,7 @@
 #include "ruby/thread.h"
 #include "ruby/util.h"
 #include "internal.h"
+#include "id.h"
 
 #ifdef HAVE_STRINGS_H
 #include <strings.h>
@@ -1544,7 +1545,7 @@ rb_big_cmp(VALUE x, VALUE y)
         return rb_integer_float_cmp(x, y);
 
       default:
-	return rb_num_coerce_cmp(x, y, rb_intern("<=>"));
+	return rb_num_coerce_cmp(x, y, idCmp);
     }
 
     if (RBIGNUM_SIGN(x) > RBIGNUM_SIGN(y)) return INT2FIX(1);
@@ -1591,10 +1592,10 @@ big_op(VALUE x, VALUE y, enum big_op_t op)
 	{
 	    ID id = 0;
 	    switch (op) {
-		case big_op_gt: id = '>'; break;
-		case big_op_ge: id = rb_intern(">="); break;
-		case big_op_lt: id = '<'; break;
-		case big_op_le: id = rb_intern("<="); break;
+		case big_op_gt: id = idGT; break;
+		case big_op_ge: id = idGE; break;
+		case big_op_lt: id = idLT; break;
+		case big_op_le: id = idLE; break;
 	    }
 	    return rb_num_coerce_relop(x, y, id);
 	}
@@ -2900,7 +2901,7 @@ rb_big_divide(VALUE x, VALUE y, ID op)
 VALUE
 rb_big_div(VALUE x, VALUE y)
 {
-    return rb_big_divide(x, y, '/');
+    return rb_big_divide(x, y, idDIV);
 }
 
 /*
@@ -3159,7 +3160,7 @@ rb_big_pow(VALUE x, VALUE y)
       case T_FLOAT:
 	d = RFLOAT_VALUE(y);
 	if ((!RBIGNUM_SIGN(x) && !BIGZEROP(x)) && d != round(d))
-	    return rb_funcall(rb_complex_raw1(x), rb_intern("**"), 1, y);
+	    return rb_funcall(rb_complex_raw1(x), idPow, 1, y);
 	break;
 
       case T_BIGNUM:
@@ -3171,7 +3172,7 @@ rb_big_pow(VALUE x, VALUE y)
 	yy = FIX2LONG(y);
 
 	if (yy < 0)
-	    return rb_funcall(rb_rational_raw1(x), rb_intern("**"), 1, y);
+	    return rb_funcall(rb_rational_raw1(x), idPow, 1, y);
 	else {
 	    VALUE z = 0;
 	    SIGNED_VALUE mask;
@@ -3196,7 +3197,7 @@ rb_big_pow(VALUE x, VALUE y)
 	break;
 
       default:
-	return rb_num_coerce_bin(x, y, rb_intern("**"));
+	return rb_num_coerce_bin(x, y, idPow);
     }
     return DBL2NUM(pow(rb_big2dbl(x), d));
 }
diff --git complex.c complex.c
index 2efe6a4..b19ecf2 100644
--- complex.c
+++ complex.c
@@ -22,7 +22,7 @@ static ID id_abs, id_abs2, id_arg, id_cmp, id_conj, id_convert,
     id_denominator, id_divmod, id_eqeq_p, id_expt, id_fdiv,  id_floor,
     id_idiv, id_imag, id_inspect, id_negate, id_numerator, id_quo,
     id_real, id_real_p, id_to_f, id_to_i, id_to_r, id_to_s,
-    id_i_real, id_i_imag;
+    id_i_real, id_i_imag, id_PI;
 
 #define f_boolcast(x) ((x) ? Qtrue : Qfalse)
 
@@ -1972,8 +1972,6 @@ numeric_abs2(VALUE self)
     return f_mul(self, self);
 }
 
-#define id_PI rb_intern("PI")
-
 /*
  * call-seq:
  *    num.arg    ->  0 or float
@@ -2112,6 +2110,7 @@ Init_Complex(void)
     id_to_s = rb_intern("to_s");
     id_i_real = rb_intern("@real");
     id_i_imag = rb_intern("@image"); /* @image, not @imag */
+    id_PI = rb_intern("PI");
 
     rb_cComplex = rb_define_class("Complex", rb_cNumeric);
 
diff --git numeric.c numeric.c
index c2562f4..0634d21 100644
--- numeric.c
+++ numeric.c
@@ -986,11 +986,11 @@ flo_pow(VALUE x, VALUE y)
 	    double dx = RFLOAT_VALUE(x);
 	    double dy = RFLOAT_VALUE(y);
 	    if (dx < 0 && dy != round(dy))
-		return rb_funcall(rb_complex_raw1(x), rb_intern("**"), 1, y);
+		return rb_funcall(rb_complex_raw1(x), idPow, 1, y);
 	    return DBL2NUM(pow(dx, dy));
 	}
       default:
-	return rb_num_coerce_bin(x, y, rb_intern("**"));
+	return rb_num_coerce_bin(x, y, idPow);
     }
 }
 
@@ -1148,7 +1148,7 @@ flo_cmp(VALUE x, VALUE y)
 	    if (a > 0.0) return INT2FIX(1);
 	    return INT2FIX(-1);
 	}
-	return rb_num_coerce_cmp(x, y, rb_intern("<=>"));
+	return rb_num_coerce_cmp(x, y, idCmp);
     }
     return rb_dbl_cmp(a, b);
 }
@@ -1228,7 +1228,7 @@ flo_ge(VALUE x, VALUE y)
 	break;
 
       default:
-	return rb_num_coerce_relop(x, y, rb_intern(">="));
+	return rb_num_coerce_relop(x, y, idGE);
     }
 #if defined(_MSC_VER) && _MSC_VER < 1300
     if (isnan(a)) return Qfalse;
@@ -1311,7 +1311,7 @@ flo_le(VALUE x, VALUE y)
 	break;
 
       default:
-	return rb_num_coerce_relop(x, y, rb_intern("<="));
+	return rb_num_coerce_relop(x, y, idLE);
     }
 #if defined(_MSC_VER) && _MSC_VER < 1300
     if (isnan(a)) return Qfalse;
@@ -1550,7 +1550,7 @@ int_round_0(VALUE num, int ndigits)
     h = rb_funcall(f, '/', 1, INT2FIX(2));
     r = rb_funcall(num, '%', 1, f);
     n = rb_funcall(num, '-', 1, r);
-    op = negative_int_p(num) ? rb_intern("<=") : '<';
+    op = negative_int_p(num) ? idLE : '<';
     if (!RTEST(rb_funcall(r, op, 1, h))) {
 	n = rb_funcall(n, '+', 1, f);
     }
@@ -2944,7 +2944,7 @@ fix_pow(VALUE x, VALUE y)
 	long b = FIX2LONG(y);
 
 	if (b < 0)
-	    return rb_funcall(rb_rational_raw1(x), rb_intern("**"), 1, y);
+	    return rb_funcall(rb_rational_raw1(x), idPow, 1, y);
 
 	if (b == 0) return INT2FIX(1);
 	if (b == 1) return x;
@@ -2965,7 +2965,7 @@ fix_pow(VALUE x, VALUE y)
       case T_BIGNUM:
 
 	if (negative_int_p(y))
-	    return rb_funcall(rb_rational_raw1(x), rb_intern("**"), 1, y);
+	    return rb_funcall(rb_rational_raw1(x), idPow, 1, y);
 
 	if (a == 0) return INT2FIX(0);
 	if (a == 1) return INT2FIX(1);
@@ -2984,11 +2984,11 @@ fix_pow(VALUE x, VALUE y)
 	{
 	    double dy = RFLOAT_VALUE(y);
 	    if (a < 0 && dy != round(dy))
-		return rb_funcall(rb_complex_raw1(x), rb_intern("**"), 1, y);
+		return rb_funcall(rb_complex_raw1(x), idPow, 1, y);
 	    return DBL2NUM(pow((double)a, dy));
 	}
       default:
-	return rb_num_coerce_bin(x, y, rb_intern("**"));
+	return rb_num_coerce_bin(x, y, idPow);
     }
 }
 
@@ -3042,7 +3042,7 @@ fix_cmp(VALUE x, VALUE y)
       case T_FLOAT:
         return rb_integer_float_cmp(x, y);
       default:
-	return rb_num_coerce_cmp(x, y, rb_intern("<=>"));
+	return rb_num_coerce_cmp(x, y, idCmp);
     }
 }
 
@@ -3095,7 +3095,7 @@ fix_ge(VALUE x, VALUE y)
           return rel == INT2FIX(1) || rel == INT2FIX(0) ? Qtrue : Qfalse;
         }
       default:
-	return rb_num_coerce_relop(x, y, rb_intern(">="));
+	return rb_num_coerce_relop(x, y, idGE);
     }
 }
 
@@ -3148,7 +3148,7 @@ fix_le(VALUE x, VALUE y)
           return rel == INT2FIX(-1) || rel == INT2FIX(0) ? Qtrue : Qfalse;
         }
       default:
-	return rb_num_coerce_relop(x, y, rb_intern("<="));
+	return rb_num_coerce_relop(x, y, idLE);
     }
 }
 
diff --git random.c random.c
index 50db6ec..160cf1b 100644
--- random.c
+++ random.c
@@ -60,6 +60,7 @@ The original copyright notice follows.
 */
 
 #include "ruby/ruby.h"
+#include "id.h"
 
 #include <limits.h>
 #ifdef HAVE_UNISTD_H
@@ -1256,7 +1257,7 @@ random_equal(VALUE self, VALUE other)
     if (rb_obj_class(self) != rb_obj_class(other)) return Qfalse;
     r1 = get_rnd(self);
     r2 = get_rnd(other);
-    if (!RTEST(rb_funcall2(r1->seed, rb_intern("=="), 1, &r2->seed))) return Qfalse;
+    if (!RTEST(rb_funcall2(r1->seed, idEq, 1, &r2->seed))) return Qfalse;
     if (memcmp(r1->mt.state, r2->mt.state, sizeof(r1->mt.state))) return Qfalse;
     if ((r1->mt.next - r1->mt.state) != (r2->mt.next - r2->mt.state)) return Qfalse;
     if (r1->mt.left != r2->mt.left) return Qfalse;
diff --git string.c string.c
index 4666176..5ba2941 100644
--- string.c
+++ string.c
@@ -870,7 +870,7 @@ rb_str_shared_replace(VALUE str, VALUE str2)
     ENC_CODERANGE_SET(str, cr);
 }
 
-static ID id_to_s;
+static ID id_to_s, id_to_str;
 
 VALUE
 rb_obj_as_string(VALUE obj)
@@ -2333,7 +2333,7 @@ rb_str_equal(VALUE str1, VALUE str2)
 {
     if (str1 == str2) return Qtrue;
     if (!RB_TYPE_P(str2, T_STRING)) {
-	if (!rb_respond_to(str2, rb_intern("to_str"))) {
+	if (!rb_respond_to(str2, id_to_str)) {
 	    return Qfalse;
 	}
 	return rb_equal(str2, str1);
@@ -2385,11 +2385,11 @@ rb_str_cmp_m(VALUE str1, VALUE str2)
     int result;
 
     if (!RB_TYPE_P(str2, T_STRING)) {
-	VALUE tmp = rb_check_funcall(str2, rb_intern("to_str"), 0, 0);
+	VALUE tmp = rb_check_funcall(str2, id_to_str, 0, 0);
 	if (RB_TYPE_P(tmp, T_STRING)) {
 	    result = rb_str_cmp(str1, tmp);
 	}
-	else if ((tmp = rb_check_funcall(str2, rb_intern("<=>"), 1, &str1)) ==
+	else if ((tmp = rb_check_funcall(str2, idCmp, 1, &str1)) ==
 		 Qundef) {
 	    return Qnil;
 	}
@@ -2739,7 +2739,7 @@ rb_str_match(VALUE x, VALUE y)
 
       generic:
       default:
-	return rb_funcall(y, rb_intern("=~"), 1, x);
+	return rb_funcall(y, idEqTilde, 1, x);
     }
 }
 
@@ -3141,7 +3141,7 @@ rb_str_upto(int argc, VALUE *argv, VALUE beg)
 	    }
 	}
 	else {
-	    ID op = excl ? '<' : rb_intern("<=");
+	    ID op = excl ? '<' : idLE;
 	    VALUE args[2], fmt = rb_obj_freeze(rb_usascii_str_new_cstr("%.*d"));
 
 	    args[0] = INT2FIX(width);
@@ -7257,7 +7257,7 @@ rb_str_sum(int argc, VALUE *argv, VALUE str)
                 sum = rb_funcall(sum, '+', 1, LONG2FIX(sum0));
             }
 
-            mod = rb_funcall(INT2FIX(1), rb_intern("<<"), 1, INT2FIX(bits));
+            mod = rb_funcall(INT2FIX(1), idLTLT, 1, INT2FIX(bits));
             mod = rb_funcall(mod, '-', 1, INT2FIX(1));
             sum = rb_funcall(sum, '&', 1, mod);
         }
@@ -8261,6 +8261,7 @@ Init_String(void)
     rb_define_method(rb_cString, "ascii_only?", rb_str_is_ascii_only_p, 0);
 
     id_to_s = rb_intern("to_s");
+    id_to_str = rb_intern("to_str");
 
     rb_fs = Qnil;
     rb_define_variable("$;", &rb_fs);
diff --git thread.c thread.c
index b1127a7..11376a2 100644
--- thread.c
+++ thread.c
@@ -63,6 +63,8 @@
 VALUE rb_cMutex;
 VALUE rb_cThreadShield;
 
+static ID id_immediate, id_on_blocking, id_defer;
+
 static void sleep_timeval(rb_thread_t *th, struct timeval time, int spurious_check);
 static void sleep_wait_for_interrupt(rb_thread_t *th, double sleepsec, int spurious_check);
 static void sleep_forever(rb_thread_t *th, int nodeadlock, int spurious_check);
@@ -1457,15 +1459,14 @@ rb_threadptr_async_errinfo_check_mask(rb_thread_t *th, VALUE err)
 	    VALUE klass = ancestors_ptr[j];
 	    VALUE sym;
 
-	    /* TODO: remove rb_intern() */
 	    if ((sym = rb_hash_aref(mask, klass)) != Qnil) {
-		if (sym == ID2SYM(rb_intern("immediate"))) {
+		if (sym == ID2SYM(id_immediate)) {
 		    return INTERRUPT_IMMEDIATE;
 		}
-		else if (sym == ID2SYM(rb_intern("on_blocking"))) {
+		else if (sym == ID2SYM(id_on_blocking)) {
 		    return INTERRUPT_ON_BLOCKING;
 		}
-		else if (sym == ID2SYM(rb_intern("defer"))) {
+		else if (sym == ID2SYM(id_defer)) {
 		    return INTERRUPT_DEFER;
 		}
 		else {
@@ -1556,9 +1557,9 @@ rb_threadptr_async_errinfo_active_p(rb_thread_t *th)
 static int
 async_interrupt_timing_arg_check_i(VALUE key, VALUE val)
 {
-    VALUE immediate = ID2SYM(rb_intern("immediate"));
-    VALUE on_blocking = ID2SYM(rb_intern("on_blocking"));
-    VALUE defer = ID2SYM(rb_intern("defer"));
+    VALUE immediate = ID2SYM(id_immediate);
+    VALUE on_blocking = ID2SYM(id_on_blocking);
+    VALUE defer = ID2SYM(id_defer);
 
     if (val != immediate && val != on_blocking && val != defer) {
 	rb_raise(rb_eArgError, "unknown mask signature");
@@ -4876,6 +4877,9 @@ Init_Thread(void)
     rb_define_method(rb_cMutex, "synchronize", rb_mutex_synchronize_m, 0);
 
     recursive_key = rb_intern("__recursive_key__");
+    id_immediate = rb_intern("immediate");
+    id_on_blocking = rb_intern("on_blocking");
+    id_defer = rb_intern("defer");
     rb_eThreadError = rb_define_class("ThreadError", rb_eStandardError);
 
     /* init thread core */
diff --git time.c time.c
index 846ff3d..707d5eb 100644
--- time.c
+++ time.c
@@ -29,8 +29,9 @@
 
 #include "timev.h"
 
-static ID id_divmod, id_mul, id_submicro, id_nano_num, id_nano_den, id_offset;
-static ID id_eq, id_ne, id_quo, id_div, id_cmp, id_lshift;
+static ID id_divmod, id_mul, id_submicro, id_nano_num, id_nano_den, id_offset,
+	id_eq, id_ne, id_quo, id_div, id_cmp, id_lshift, id_to_r, id_to_str,
+	id_dst, id_std;
 
 #define NDIV(x,y) (-(-((x)+1)/(y))-1)
 #define NMOD(x,y) ((y)-(-((x)+1)%(y))-1)
@@ -678,8 +679,8 @@ num_exact(VALUE v)
         goto typeerror;
 
       default:
-        if ((tmp = rb_check_funcall(v, rb_intern("to_r"), 0, NULL)) != Qundef) {
-            if (rb_respond_to(v, rb_intern("to_str"))) goto typeerror;
+        if ((tmp = rb_check_funcall(v, id_to_r, 0, NULL)) != Qundef) {
+            if (rb_respond_to(v, id_to_str)) goto typeerror;
             v = tmp;
             break;
         }
@@ -2186,9 +2187,9 @@ time_init_1(int argc, VALUE *argv, VALUE time)
     vtm.utc_offset = Qnil;
     if (!NIL_P(v[6])) {
         VALUE arg = v[6];
-        if (arg == ID2SYM(rb_intern("dst")))
+        if (arg == ID2SYM(id_dst))
             vtm.isdst = 1;
-        else if (arg == ID2SYM(rb_intern("std")))
+        else if (arg == ID2SYM(id_std))
             vtm.isdst = 0;
         else
             vtm.utc_offset = utc_offset_arg(arg);
@@ -3361,7 +3362,7 @@ time_cmp(VALUE time1, VALUE time2)
     else {
 	VALUE tmp;
 
-	tmp = rb_funcall(time2, rb_intern("<=>"), 1, time1);
+	tmp = rb_funcall(time2, id_cmp, 1, time1);
 	if (NIL_P(tmp)) return Qnil;
 
 	n = -rb_cmpint(tmp, time1, time2);
@@ -4955,6 +4956,10 @@ Init_Time(void)
     id_nano_num = rb_intern("nano_num");
     id_nano_den = rb_intern("nano_den");
     id_offset = rb_intern("offset");
+    id_to_r = rb_intern("to_r");
+    id_to_str = rb_intern("to_str");
+    id_dst = rb_intern("dst");
+    id_std = rb_intern("std");
 
     rb_cTime = rb_define_class("Time", rb_cObject);
     rb_include_module(rb_cTime, rb_mComparable);
diff --git vm_eval.c vm_eval.c
index f0b0c50..8688e36 100644
--- vm_eval.c
+++ vm_eval.c
@@ -19,6 +19,8 @@ static VALUE vm_exec(rb_thread_t *th);
 static void vm_set_eval_stack(rb_thread_t * th, VALUE iseqval, const NODE *cref, rb_block_t *base_block);
 static int vm_collect_local_variables_in_heap(rb_thread_t *th, VALUE *dfp, VALUE ary);
 
+static ID id_message, id_call;
+
 /* vm_backtrace.c */
 VALUE vm_backtrace_str_ary(rb_thread_t *th, int lev, int n);
 
@@ -592,7 +594,7 @@ make_no_method_exception(VALUE exc, const char *format, VALUE obj, int argc, con
     if (!format) {
 	format = "undefined method `%s' for %s";
     }
-    mesg = rb_const_get(exc, rb_intern("message"));
+    mesg = rb_const_get(exc, id_message);
     if (rb_method_basic_definition_p(CLASS_OF(mesg), '!')) {
 	args[n++] = rb_name_err_mesg_new(mesg, rb_str_new2(format), obj, argv[0]);
     }
@@ -1391,7 +1393,7 @@ rb_eval_cmd(VALUE cmd, VALUE arg, int level)
 	PUSH_TAG();
 	rb_set_safe_level_force(level);
 	if ((state = EXEC_TAG()) == 0) {
-	    val = rb_funcall2(cmd, rb_intern("call"), RARRAY_LENINT(arg),
+	    val = rb_funcall2(cmd, id_call, RARRAY_LENINT(arg),
 			      RARRAY_PTR(arg));
 	}
 	POP_TAG();
@@ -1876,6 +1878,9 @@ Init_vm_eval(void)
     rb_define_method(rb_cBasicObject, "instance_exec", rb_obj_instance_exec, -1);
     rb_define_private_method(rb_cBasicObject, "method_missing", rb_method_missing, -1);
 
+    id_message = rb_intern("message");
+    id_call = rb_intern("call");
+
 #if 1
     rb_add_method(rb_cBasicObject, rb_intern("__send__"),
 		  VM_METHOD_TYPE_OPTIMIZED, (void *)OPTIMIZED_METHOD_TYPE_SEND, 0);