diff --git a/ChangeLog b/ChangeLog
index 585a6c8dda..93abe653ae 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+Wed Oct 27 02:12:10 2010  Yusuke Endoh  <mame@tsg.ne.jp>
+
+	* object.c (Init_Object), constant.h, variable.c
+	  (rb_mod_private_constant, rb_mod_public_constant,
+	  set_const_visibility, rb_const_get_0): add Module#public_constant
+	  and private_constant.  [ruby-dev:39685][ruby-core:32698]
+
+	* test/ruby/test_module.rb: add tests for above.
+
 Wed Oct 27 02:02:54 2010  Yusuke Endoh  <mame@tsg.ne.jp>
 
 	* class.c, constant.h, gc.c, method.h, object.c, variable.c,
diff --git a/constant.h b/constant.h
index 5c04b01f41..d347ea19f5 100644
--- a/constant.h
+++ b/constant.h
@@ -21,6 +21,8 @@ typedef struct rb_const_entry_struct {
     VALUE value;            /* should be mark */
 } rb_const_entry_t;
 
+VALUE rb_mod_private_constant(int argc, VALUE *argv, VALUE obj);
+VALUE rb_mod_public_constant(int argc, VALUE *argv, VALUE obj);
 void rb_free_const_table(st_table *tbl);
 
 #endif /* CONSTANT_H */
diff --git a/object.c b/object.c
index 30f308bf60..252af17681 100644
--- a/object.c
+++ b/object.c
@@ -2648,6 +2648,8 @@ Init_Object(void)
     rb_define_method(rb_cModule, "class_variable_get", rb_mod_cvar_get, 1);
     rb_define_method(rb_cModule, "class_variable_set", rb_mod_cvar_set, 2);
     rb_define_method(rb_cModule, "class_variable_defined?", rb_mod_cvar_defined, 1);
+    rb_define_method(rb_cModule, "public_constant", rb_mod_public_constant, -1);
+    rb_define_method(rb_cModule, "private_constant", rb_mod_private_constant, -1);
 
     rb_define_method(rb_cClass, "allocate", rb_obj_alloc, 0);
     rb_define_method(rb_cClass, "new", rb_class_new_instance, -1);
diff --git a/test/ruby/test_module.rb b/test/ruby/test_module.rb
index e5f1e69749..87a4905e3c 100644
--- a/test/ruby/test_module.rb
+++ b/test/ruby/test_module.rb
@@ -939,4 +939,24 @@ class TestModule < Test::Unit::TestCase
     assert_nothing_raised(bug3406) {c.x = 1}
     assert_equal(1, c.x, bug3406)
   end
+
+  def test_private_constant
+    c = Class.new
+    c.const_set(:FOO, "foo")
+    assert_equal("foo", c::FOO)
+    c.private_constant(:FOO)
+    assert_raise(NameError) { c::FOO }
+    assert_equal("foo", c.class_eval("FOO"))
+  end
+
+  def test_public_constant
+    c = Class.new
+    c.const_set(:FOO, "foo")
+    assert_equal("foo", c::FOO)
+    c.private_constant(:FOO)
+    assert_raise(NameError) { c::FOO }
+    assert_equal("foo", c.class_eval("FOO"))
+    c.public_constant(:FOO)
+    assert_equal("foo", c::FOO)
+  end
 end
diff --git a/variable.c b/variable.c
index bc7a692f12..b372eb2591 100644
--- a/variable.c
+++ b/variable.c
@@ -1586,7 +1586,11 @@ rb_const_get_0(VALUE klass, ID id, int exclude, int recurse)
 	VALUE am = 0;
 	st_data_t data;
 	while (RCLASS_CONST_TBL(tmp) && st_lookup(RCLASS_CONST_TBL(tmp), (st_data_t)id, &data)) {
-	    value = ((rb_const_entry_t*)data)->value;
+	    rb_const_entry_t *ce = (rb_const_entry_t *)data;
+	    if (ce->flag == CONST_PRIVATE) {
+		rb_name_error(id, "private constant %s::%s referenced", rb_class2name(klass), rb_id2name(id));
+	    }
+	    value = ce->value;
 	    if (value == Qundef) {
 		if (am == tmp) break;
 		am = tmp;
@@ -1882,6 +1886,57 @@ rb_define_global_const(const char *name, VALUE val)
     rb_define_const(rb_cObject, name, val);
 }
 
+static void
+set_const_visibility(VALUE mod, int argc, VALUE *argv, rb_const_flag_t flag)
+{
+    int i;
+    st_data_t v;
+    ID id;
+
+    if (rb_safe_level() >= 4 && !OBJ_UNTRUSTED(mod)) {
+	rb_raise(rb_eSecurityError,
+		 "Insecure: can't change method visibility");
+    }
+
+    for (i = 0; i < argc; i++) {
+	id = rb_to_id(argv[i]);
+	if (RCLASS_CONST_TBL(mod) && st_lookup(RCLASS_CONST_TBL(mod), (st_data_t)id, &v)) {
+	    ((rb_const_entry_t*)v)->flag = flag;
+	    return;
+	}
+	rb_name_error(id, "constant %s::%s not defined", rb_class2name(mod), rb_id2name(id));
+    }
+    rb_clear_cache_by_class(mod);
+}
+
+/*
+ *  call-seq:
+ *     mod.private_constant(symbol, ...)    => mod
+ *
+ *  Makes a list of existing constants private.
+ */
+
+VALUE
+rb_mod_private_constant(int argc, VALUE *argv, VALUE obj)
+{
+    set_const_visibility(obj, argc, argv, CONST_PRIVATE);
+    return obj;
+}
+
+/*
+ *  call-seq:
+ *     mod.public_constant(symbol, ...)    => mod
+ *
+ *  Makes a list of existing constants public.
+ */
+
+VALUE
+rb_mod_public_constant(int argc, VALUE *argv, VALUE obj)
+{
+    set_const_visibility(obj, argc, argv, CONST_PUBLIC);
+    return obj;
+}
+
 static VALUE
 original_module(VALUE c)
 {