diff --git a/ChangeLog b/ChangeLog
index 398fedcd4c..ec6cd5f7ee 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat Jun 18 12:05:08 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* include/ruby/backward/classext.h: for evil gems.  fixed #4803
+
 Sat Jun 18 11:12:13 2011  Tanaka Akira  <akr@fsij.org>
 
 	* common.mk: update dependencies.
diff --git a/include/ruby/backward/classext.h b/include/ruby/backward/classext.h
new file mode 100644
index 0000000000..615e6f6858
--- /dev/null
+++ b/include/ruby/backward/classext.h
@@ -0,0 +1,18 @@
+#if   defined __GNUC__
+#warning use of RClass internals is deprecated
+#elif defined _MSC_VER || defined __BORLANDC__
+#pragma message("warning: use of RClass internals is deprecated")
+#endif
+
+#ifndef RUBY_BACKWARD_CLASSEXT_H
+#define RUBY_BACKWARD_CLASSEXT_H 1
+
+typedef struct rb_deprecated_classext_struct {
+    VALUE super;
+} rb_deprecated_classext_t;
+
+#undef RCLASS_SUPER(c)
+#define RCLASS_EXT(c) ((rb_deprecated_classext_t *)RCLASS(c)->ptr)
+#define RCLASS_SUPER(c) (RCLASS_EXT(c)->super)
+
+#endif	/* RUBY_BACKWARD_CLASSEXT_H */
diff --git a/include/ruby/intern.h b/include/ruby/intern.h
index bc3c4491cd..582fa8801d 100644
--- a/include/ruby/intern.h
+++ b/include/ruby/intern.h
@@ -519,6 +519,7 @@ VALUE rb_obj_class(VALUE);
 VALUE rb_class_real(VALUE);
 VALUE rb_class_inherited_p(VALUE, VALUE);
 VALUE rb_class_superclass(VALUE);
+VALUE rb_class_get_superclass(VALUE);
 VALUE rb_convert_type(VALUE,int,const char*,const char*);
 VALUE rb_check_convert_type(VALUE,int,const char*,const char*);
 VALUE rb_check_to_integer(VALUE, const char *);
diff --git a/include/ruby/ruby.h b/include/ruby/ruby.h
index 38bfe3f56c..340bea102a 100644
--- a/include/ruby/ruby.h
+++ b/include/ruby/ruby.h
@@ -617,11 +617,7 @@ struct RClass {
     struct st_table *m_tbl;
     struct st_table *iv_index_tbl;
 };
-#define RCLASS_IV_TBL(c) (RCLASS(c)->ptr->iv_tbl)
-#define RCLASS_CONST_TBL(c) (RCLASS(c)->ptr->const_tbl)
-#define RCLASS_M_TBL(c) (RCLASS(c)->m_tbl)
-#define RCLASS_SUPER(c) (RCLASS(c)->ptr->super)
-#define RCLASS_IV_INDEX_TBL(c) (RCLASS(c)->iv_index_tbl)
+#define RCLASS_SUPER(c) rb_class_get_superclass(c)
 #define RMODULE_IV_TBL(m) RCLASS_IV_TBL(m)
 #define RMODULE_CONST_TBL(m) RCLASS_CONST_TBL(m)
 #define RMODULE_M_TBL(m) RCLASS_M_TBL(m)
diff --git a/internal.h b/internal.h
index c95c5f87f6..2cc3b529df 100644
--- a/internal.h
+++ b/internal.h
@@ -19,12 +19,24 @@ extern "C" {
 #endif
 #endif
 
+struct rb_deprecated_classext_struct {
+    char conflict[sizeof(VALUE) * 3];
+};
+
 struct rb_classext_struct {
     VALUE super;
     struct st_table *iv_tbl;
     struct st_table *const_tbl;
 };
 
+#undef RCLASS_SUPER
+#define RCLASS_EXT(c) (RCLASS(c)->ptr)
+#define RCLASS_SUPER(c) (RCLASS_EXT(c)->super)
+#define RCLASS_IV_TBL(c) (RCLASS_EXT(c)->iv_tbl)
+#define RCLASS_CONST_TBL(c) (RCLASS_EXT(c)->const_tbl)
+#define RCLASS_M_TBL(c) (RCLASS(c)->m_tbl)
+#define RCLASS_IV_INDEX_TBL(c) (RCLASS(c)->iv_index_tbl)
+
 struct vtm; /* defined by timev.h */
 
 /* bignum.c */
diff --git a/object.c b/object.c
index 62edcb6c47..e6de509d47 100644
--- a/object.c
+++ b/object.c
@@ -1668,6 +1668,12 @@ rb_class_superclass(VALUE klass)
     return super;
 }
 
+VALUE
+rb_class_get_superclass(VALUE klass)
+{
+    return RCLASS_SUPER(klass);
+}
+
 /*
  *  call-seq:
  *     attr_reader(symbol, ...)    -> nil