diff --git a/class.c b/class.c
index dfb058a..c3f1948 100644
--- a/class.c
+++ b/class.c
@@ -26,6 +26,7 @@
 #include "ruby/ruby.h"
 #include "ruby/st.h"
 #include "method.h"
+#include "constant.h"
 #include "vm_core.h"
 #include <ctype.h>
 
@@ -140,6 +141,14 @@ clone_method(ID mid, const rb_method_entry_t *me, struct clone_method_data *data
     return ST_CONTINUE;
 }
 
+static int
+clone_const(ID key, const rb_const_entry_t *ce, st_table *tbl)
+{
+    rb_const_entry_t *nce = ALLOC(rb_const_entry_t);
+    *nce = *ce;
+    st_insert(tbl, key, (st_data_t)nce);
+}
+
 /* :nodoc: */
 VALUE
 rb_mod_init_copy(VALUE clone, VALUE orig)
@@ -163,15 +172,15 @@ rb_mod_init_copy(VALUE clone, VALUE orig)
     }
     if (RCLASS_CONST_TBL(clone)) {
 	if (RCLASS_CONST_TBL(clone)) {
-	    st_free_table(RCLASS_CONST_TBL(clone));
+	    rb_free_const_table(RCLASS_CONST_TBL(clone));
 	}
-	RCLASS_CONST_TBL(clone) = st_copy(RCLASS_CONST_TBL(orig));
+	RCLASS_CONST_TBL(clone) = st_init_numtable();
+	st_foreach(RCLASS_CONST_TBL(orig), clone_const, (st_data_t)RCLASS_CONST_TBL(clone));
     }
     if (RCLASS_M_TBL(orig)) {
 	struct clone_method_data data;
 
 	if (RCLASS_M_TBL(clone)) {
-	    extern void rb_free_m_table(st_table *tbl);
 	    rb_free_m_table(RCLASS_M_TBL(clone));
 	}
 	data.tbl = RCLASS_M_TBL(clone) = st_init_numtable();
@@ -220,7 +229,8 @@ rb_singleton_class_clone(VALUE obj)
 	    RCLASS_IV_TBL(clone) = st_copy(RCLASS_IV_TBL(klass));
 	}
 	if (RCLASS_CONST_TBL(klass)) {
-	    RCLASS_CONST_TBL(clone) = st_copy(RCLASS_CONST_TBL(klass));
+	    RCLASS_CONST_TBL(clone) = st_init_numtable();
+	    st_foreach(RCLASS_CONST_TBL(obj), clone_const, (st_data_t)RCLASS_CONST_TBL(clone));
 	}
 	RCLASS_M_TBL(clone) = st_init_numtable();
 	data.tbl = RCLASS_M_TBL(clone);
diff --git a/constant.h b/constant.h
new file mode 100644
index 0000000..5c04b01
--- /dev/null
+++ b/constant.h
@@ -0,0 +1,26 @@
+/**********************************************************************
+
+  constant.h -
+
+  $Author$
+  created at: Sun Nov 15 00:09:33 2009
+
+  Copyright (C) 2009 Yusuke Endoh
+
+**********************************************************************/
+#ifndef CONSTANT_H
+#define CONSTANT_H
+
+typedef enum {
+    CONST_PUBLIC    = 0x00,
+    CONST_PRIVATE   = 0x01,
+} rb_const_flag_t;
+
+typedef struct rb_const_entry_struct {
+    rb_const_flag_t flag;
+    VALUE value;            /* should be mark */
+} rb_const_entry_t;
+
+void rb_free_const_table(st_table *tbl);
+
+#endif /* CONSTANT_H */
diff --git a/gc.c b/gc.c
index 45952ee..98712cb 100644
--- a/gc.c
+++ b/gc.c
@@ -19,6 +19,7 @@
 #include "eval_intern.h"
 #include "vm_core.h"
 #include "gc.h"
+#include "constant.h"
 #include <stdio.h>
 #include <setjmp.h>
 #include <sys/types.h>
@@ -1466,6 +1467,38 @@ rb_free_m_table(st_table *tbl)
     st_free_table(tbl);
 }
 
+static int
+mark_const_entry_i(ID key, const rb_const_entry_t *ce, st_data_t data)
+{
+    struct mark_tbl_arg *arg = (void*)data;
+    gc_mark(arg->objspace, ce->value, arg->lev);
+    return ST_CONTINUE;
+}
+
+static void
+mark_const_tbl(rb_objspace_t *objspace, st_table *tbl, int lev)
+{
+    struct mark_tbl_arg arg;
+    if (!tbl) return;
+    arg.objspace = objspace;
+    arg.lev = lev;
+    st_foreach(tbl, mark_const_entry_i, (st_data_t)&arg);
+}
+
+static int
+free_const_entry_i(ID key, rb_const_entry_t *ce, st_data_t data)
+{
+    xfree(ce);
+    return ST_CONTINUE;
+}
+
+void
+rb_free_const_table(st_table *tbl)
+{
+    st_foreach(tbl, free_const_entry_i, 0);
+    st_free_table(tbl);
+}
+
 void
 rb_mark_tbl(st_table *tbl)
 {
@@ -1678,7 +1711,7 @@ gc_mark_children(rb_objspace_t *objspace, VALUE ptr, int lev)
       case T_MODULE:
 	mark_m_tbl(objspace, RCLASS_M_TBL(obj), lev);
 	mark_tbl(objspace, RCLASS_IV_TBL(obj), lev);
-	mark_tbl(objspace, RCLASS_CONST_TBL(obj), lev);
+	mark_const_tbl(objspace, RCLASS_CONST_TBL(obj), lev);
 	ptr = RCLASS_SUPER(obj);
 	goto again;
 
@@ -1994,7 +2027,7 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
 	    st_free_table(RCLASS_IV_TBL(obj));
 	}
 	if (RCLASS_CONST_TBL(obj)) {
-	    st_free_table(RCLASS_CONST_TBL(obj));
+	    rb_free_const_table(RCLASS_CONST_TBL(obj));
 	}
 	if (RCLASS_IV_INDEX_TBL(obj)) {
 	    st_free_table(RCLASS_IV_INDEX_TBL(obj));
diff --git a/method.h b/method.h
index 177d7bc..466a561 100644
--- a/method.h
+++ b/method.h
@@ -85,5 +85,6 @@ rb_method_entry_t *rb_get_method_entry(VALUE klass, ID id);
 int rb_method_entry_arity(const rb_method_entry_t *me);
 void rb_gc_mark_method_entry(const rb_method_entry_t *me);
 void rb_free_method_entry(rb_method_entry_t *me);
+void rb_free_m_table(st_table *tbl);
 
 #endif /* METHOD_H */
diff --git a/object.c b/object.c
index b110767..6f94fe6 100644
--- a/object.c
+++ b/object.c
@@ -19,6 +19,7 @@
 #include <ctype.h>
 #include <math.h>
 #include <float.h>
+#include "constant.h"
 
 VALUE rb_cBasicObject;
 VALUE rb_mKernel;
@@ -200,7 +201,7 @@ init_copy(VALUE dest, VALUE obj)
 	    RCLASS_IV_TBL(dest) = 0;
 	}
 	if (RCLASS_CONST_TBL(dest)) {
-	    st_free_table(RCLASS_CONST_TBL(dest));
+	    rb_free_const_table(RCLASS_CONST_TBL(dest));
 	    RCLASS_CONST_TBL(dest) = 0;
 	}
 	if (RCLASS_IV_TBL(obj)) {
diff --git a/variable.c b/variable.c
index a893564..4194c8b 100644
--- a/variable.c
+++ b/variable.c
@@ -16,6 +16,7 @@
 #include "ruby/util.h"
 #include "ruby/encoding.h"
 #include "node.h"
+#include "constant.h"
 
 void rb_vm_change_state(void);
 void rb_vm_inc_const_missing_count(void);
@@ -71,8 +72,9 @@ fc_path(struct fc_result *fc, ID name)
 }
 
 static int
-fc_i(ID key, VALUE value, struct fc_result *res)
+fc_i(ID key, rb_const_entry_t *ce, struct fc_result *res)
 {
+    VALUE value = ce->value;
     if (!rb_is_const_id(key)) return ST_CONTINUE;
 
     if (value == res->klass) {
@@ -1398,7 +1400,7 @@ rb_autoload(VALUE mod, ID id, const char *file)
 	rb_raise(rb_eArgError, "empty file name");
     }
 
-    if ((tbl = RCLASS_CONST_TBL(mod)) && st_lookup(tbl, (st_data_t)id, &av) && (VALUE)av != Qundef)
+    if ((tbl = RCLASS_CONST_TBL(mod)) && st_lookup(tbl, (st_data_t)id, &av) && ((rb_const_entry_t*)av)->value != Qundef)
 	return;
 
     rb_const_set(mod, id, Qundef);
@@ -1421,8 +1423,11 @@ static NODE*
 autoload_delete(VALUE mod, ID id)
 {
     st_data_t val, load = 0, n = id;
+    rb_const_entry_t *ce;
 
-    st_delete(RCLASS_CONST_TBL(mod), &n, 0);
+    st_delete(RCLASS_CONST_TBL(mod), &n, &val);
+    ce = (rb_const_entry_t*)val;
+    if (ce) xfree(ce);
     if (st_lookup(RCLASS_IV_TBL(mod), (st_data_t)autoload, &val)) {
 	struct st_table *tbl = check_autoload_table((VALUE)val);
 
@@ -1431,6 +1436,8 @@ autoload_delete(VALUE mod, ID id)
 	if (tbl->num_entries == 0) {
 	    n = autoload;
 	    st_delete(RCLASS_CONST_TBL(mod), &n, &val);
+	    ce = (rb_const_entry_t*)val;
+	    if (ce) xfree(ce);
 	}
     }
 
@@ -1490,7 +1497,7 @@ autoload_node_id(VALUE mod, ID id)
     struct st_table *tbl = RCLASS_CONST_TBL(mod);
     st_data_t val;
 
-    if (!tbl || !st_lookup(tbl, (st_data_t)id, &val) || (VALUE)val != Qundef) {
+    if (!tbl || !st_lookup(tbl, (st_data_t)id, &val) || ((rb_const_entry_t*)val)->value != Qundef) {
 	return 0;
     }
     return 1;
@@ -1528,6 +1535,7 @@ rb_autoload_p(VALUE mod, ID id)
 static VALUE
 rb_const_get_0(VALUE klass, ID id, int exclude, int recurse)
 {
+    st_data_t v;
     VALUE value, tmp;
     int mod_retry = 0;
 
@@ -1535,7 +1543,8 @@ rb_const_get_0(VALUE klass, ID id, int exclude, int recurse)
   retry:
     while (RTEST(tmp)) {
 	VALUE am = 0;
-	while (RCLASS_CONST_TBL(tmp) && st_lookup(RCLASS_CONST_TBL(tmp), (st_data_t)id, &value)) {
+	while (RCLASS_CONST_TBL(tmp) && st_lookup(RCLASS_CONST_TBL(tmp), (st_data_t)id, &v)) {
+	    value = ((rb_const_entry_t*)v)->value;
 	    if (value == Qundef) {
 		if (am == tmp) break;
 		am = tmp;
@@ -1614,16 +1623,17 @@ rb_mod_remove_const(VALUE mod, VALUE name)
 
     rb_vm_change_state();
 
-    val = (VALUE)v;
+    val = ((rb_const_entry_t*)v)->value;
     if (val == Qundef) {
 	autoload_delete(mod, id);
 	val = Qnil;
     }
+    xfree((rb_const_entry_t*)v);
     return val;
 }
 
 static int
-sv_i(ID key, VALUE value, st_table *tbl)
+sv_i(ID key, rb_const_entry_t *ce, st_table *tbl)
 {
     if (rb_is_const_id(key)) {
 	if (!st_lookup(tbl, (st_data_t)key, 0)) {
@@ -1727,7 +1737,7 @@ rb_const_defined_0(VALUE klass, ID id, int exclude, int recurse)
   retry:
     while (tmp) {
 	if (RCLASS_CONST_TBL(tmp) && st_lookup(RCLASS_CONST_TBL(tmp), (st_data_t)id, &value)) {
-	    if ((VALUE)value == Qundef && !autoload_node((VALUE)klass, id, 0))
+	    if (((rb_const_entry_t*)value)->value == Qundef && !autoload_node((VALUE)klass, id, 0))
 		return (int)Qfalse;
 	    return (int)Qtrue;
 	}
@@ -1778,6 +1788,8 @@ check_before_mod_set(VALUE klass, ID id, VALUE val, const char *dest)
 void
 rb_const_set(VALUE klass, ID id, VALUE val)
 {
+    rb_const_entry_t *ce;
+
     if (NIL_P(klass)) {
 	rb_raise(rb_eTypeError, "no class/module to define constant %s",
 		 rb_id2name(id));
@@ -1791,7 +1803,7 @@ rb_const_set(VALUE klass, ID id, VALUE val)
 	st_data_t value;
 
 	if (st_lookup(RCLASS_CONST_TBL(klass), (st_data_t)id, &value)) {
-	    if ((VALUE)value == Qundef)
+	    if (((rb_const_entry_t*)value)->value == Qundef)
 		autoload_delete(klass, id);
 	    else
 		rb_warn("already initialized constant %s", rb_id2name(id));
@@ -1799,7 +1811,12 @@ rb_const_set(VALUE klass, ID id, VALUE val)
     }
 
     rb_vm_change_state();
-    st_insert(RCLASS_CONST_TBL(klass), (st_data_t)id, (st_data_t)val);
+
+    ce = ALLOC(rb_const_entry_t);
+    ce->flag = CONST_PUBLIC;
+    ce->value = val;
+
+    st_insert(RCLASS_CONST_TBL(klass), (st_data_t)id, (st_data_t)ce);
 }
 
 void
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 4023962..5440525 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -11,6 +11,7 @@
 /* finish iseq array */
 #include "insns.inc"
 #include <math.h>
+#include "constant.h"
 
 /* control stack frame */
 
@@ -1126,10 +1127,12 @@ vm_get_ev_const(rb_thread_t *th, const rb_iseq_t *iseq,
 	    cref = cref->nd_next;
 
 	    if (!NIL_P(klass)) {
+		st_data_t v;
 		VALUE am = 0;
 	      search_continue:
 		if (RCLASS_CONST_TBL(klass) &&
-		    st_lookup(RCLASS_CONST_TBL(klass), id, &val)) {
+		    st_lookup(RCLASS_CONST_TBL(klass), id, &v)) {
+		    val = ((rb_const_entry_t*)v)->value;
 		    if (val == Qundef) {
 			if (am == klass) break;
 			am = klass;
