diff --git a/ChangeLog b/ChangeLog
index 57e4b407cb..8ced755d6f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Fri Dec  7 15:44:40 2007  Tanaka Akira  <akr@fsij.org>
+
+	* lib/pp.rb: use Hash for recursion check as inspect.
+
 Fri Dec  7 15:04:01 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* array.c (flatten): some performance improvements, based on a patch
diff --git a/lib/pp.rb b/lib/pp.rb
index 0202c52073..74f690e9f8 100644
--- a/lib/pp.rb
+++ b/lib/pp.rb
@@ -111,13 +111,13 @@ class PP < PrettyPrint
       end
 
       if Thread.current[:__recursive_key__][:inspect] == nil
-        Thread.current[:__recursive_key__][:inspect] = []
+        Thread.current[:__recursive_key__][:inspect] = {}
       end
 
       save = Thread.current[:__recursive_key__][:inspect]
 
       begin
-        Thread.current[:__recursive_key__][:inspect] = []
+        Thread.current[:__recursive_key__][:inspect] = {}
         yield
       ensure
         Thread.current[:__recursive_key__][:inspect] = save
@@ -130,10 +130,10 @@ class PP < PrettyPrint
       Thread.current[:__recursive_key__][:inspect].include?(id)
     end
     def push_inspect_key(id)
-      Thread.current[:__recursive_key__][:inspect] << id
+      Thread.current[:__recursive_key__][:inspect][id] = true
     end
-    def pop_inspect_key
-      Thread.current[:__recursive_key__][:inspect].pop
+    def pop_inspect_key(id)
+      Thread.current[:__recursive_key__][:inspect].delete id
     end
 
     # Adds +obj+ to the pretty printing buffer
@@ -153,7 +153,7 @@ class PP < PrettyPrint
         push_inspect_key(id)
         group {obj.pretty_print self}
       ensure
-        pop_inspect_key unless PP.sharing_detection
+        pop_inspect_key(id) unless PP.sharing_detection
       end
     end