diff --git a/ChangeLog b/ChangeLog
index 882e7a3a6035d7..6aaa59bdfa427b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+Sun Jan 15 14:09:48 2012  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* object.c (rb_inspect): raise the result is not compatible with
+	  the default external encoding. [ruby-core:42095] [Bug #5848]
+	  If the default external encoding is ASCII compatible, the encoding of
+	  inspected result must be compatible with it.
+	  If the default external encoding is ASCII incomapatible,
+	  the result must be ASCII only.
+
 Sun Jan 15 13:21:50 2012  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* ext/json/parser/parser.rl (json_string_unescape): workaround fix
diff --git a/object.c b/object.c
index 09b399923166f3..e3302d153ed1c7 100644
--- a/object.c
+++ b/object.c
@@ -368,12 +368,25 @@ rb_any_to_s(VALUE obj)
     return str;
 }
 
+/*
+ * If the default external encoding is ASCII compatible, the encoding of
+ * inspected result must be compatible with it.
+ * If the default external encoding is ASCII incomapatible,
+ * the result must be ASCII only.
+ */
 VALUE
 rb_inspect(VALUE obj)
 {
-    VALUE s = rb_obj_as_string(rb_funcall(obj, id_inspect, 0, 0));
-    rb_enc_check(rb_enc_default_external(), s);
-    return s;
+    VALUE str = rb_obj_as_string(rb_funcall(obj, id_inspect, 0, 0));
+    rb_encoding *ext = rb_default_external_encoding();
+    if (!rb_enc_asciicompat(ext)) {
+	if (!rb_enc_str_asciionly_p(str))
+	    rb_raise(rb_eEncCompatError, "inspected result must be ASCII only if default external encoding is ASCII incompatible");
+	return str;
+    }
+    if (rb_enc_get(str) != ext && !rb_enc_str_asciionly_p(str))
+	rb_raise(rb_eEncCompatError, "inspected result must be ASCII only or use the same encoding with default external");
+    return str;
 }
 
 static int
diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index 7c261138ba0eda..3bfb540b1356ab 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -271,6 +271,34 @@ def test_object_utf16_32_inspect
     Encoding.default_external = orig_ext
   end
 
+  def test_object_inspect_external
+    orig_v, $VERBOSE = $VERBOSE, false
+    orig_int, Encoding.default_internal = Encoding.default_internal, nil
+    orig_ext = Encoding.default_external
+    o = Object.new
+
+    Encoding.default_external = Encoding::UTF_16BE
+    def o.inspect
+      "abc"
+    end
+    assert_nothing_raised(Encoding::CompatibilityError) { [o].inspect }
+
+    def o.inspect
+      "abc".encode(Encoding.default_external)
+    end
+    assert_raise(Encoding::CompatibilityError) { [o].inspect }
+
+    Encoding.default_external = Encoding::US_ASCII
+    def o.inspect
+      "\u3042"
+    end
+    assert_raise(Encoding::CompatibilityError) { [o].inspect }
+  ensure
+    Encoding.default_internal = orig_int
+    Encoding.default_external = orig_ext
+    $VERBOSE = orig_v
+  end
+
   def test_str_dump
     [
       e("\xfe"),