diff --git a/ChangeLog b/ChangeLog
index e831da73e6..b6dcd93cb2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Mon Mar  7 22:59:39 2011  Shota Fukumori  <sorah@tubusu.net>
+
+	* lib/pstore.rb: Delete variable @transaction and fix #4474. Patch by
+	  Masaki Matsushita (Glass_saga).
+
+	* test/test_pstore.rb(test_thread_safe): Add test for #4474.
+
 Mon Mar  7 21:31:38 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* process.c (proc_setgroups): replace getgrnam() with getgrnam_r()
diff --git a/lib/pstore.rb b/lib/pstore.rb
index 65cce7b238..3114eb567f 100644
--- a/lib/pstore.rb
+++ b/lib/pstore.rb
@@ -127,21 +127,16 @@ class PStore
     if File::exist? file and not File::readable? file
       raise PStore::Error, format("file %s not readable", file)
     end
-    @transaction = false
     @filename = file
     @abort = false
     @ultra_safe = false
     @thread_safe = thread_safe
-    if @thread_safe
-      @lock = Mutex.new
-    else
-      @lock = DummyMutex.new
-    end
+    @lock = Mutex.new
   end
 
   # Raises PStore::Error if the calling code is not in a PStore#transaction.
   def in_transaction
-    raise PStore::Error, "not in transaction" unless @transaction
+    raise PStore::Error, "not in transaction" unless @lock.locked?
   end
   #
   # Raises PStore::Error if the calling code is not in a PStore#transaction or
@@ -318,10 +313,9 @@ class PStore
   #
   def transaction(read_only = false, &block)  # :yields:  pstore
     value = nil
-    raise PStore::Error, "nested transaction" if @transaction
+    raise PStore::Error, "nested transaction" if !@thread_safe && @lock.locked?
     @lock.synchronize do
       @rdonly = read_only
-      @transaction = true
       @abort = false
       file = open_and_lock_file(@filename, read_only)
       if file
@@ -347,8 +341,6 @@ class PStore
       end
     end
     value
-  ensure
-    @transaction = false
   end
 
   private
@@ -357,12 +349,6 @@ class PStore
   EMPTY_MARSHAL_DATA = Marshal.dump({})
   EMPTY_MARSHAL_CHECKSUM = Digest::MD5.digest(EMPTY_MARSHAL_DATA)
 
-  class DummyMutex
-    def synchronize
-      yield
-    end
-  end
-
   #
   # Open the specified filename (either in read-only mode or in
   # read-write mode) and lock it for reading or writing.
diff --git a/test/test_pstore.rb b/test/test_pstore.rb
index e10ce906cb..32d79ea2ba 100644
--- a/test/test_pstore.rb
+++ b/test/test_pstore.rb
@@ -71,4 +71,33 @@ class PStoreTest < Test::Unit::TestCase
       end
     end
   end
+
+  def test_thread_safe
+    assert_raise(PStore::Error) do
+      flag = false
+      Thread.new do
+        @pstore.transaction do
+          @pstore[:foo] = "bar"
+          flag = true
+          sleep 1
+        end
+      end
+      until flag; end
+      @pstore.transaction {}
+    end
+    assert_block do
+      pstore = PStore.new("pstore.tmp2.#{Process.pid}",true)
+      flag = false
+      Thread.new do
+        pstore.transaction do
+          pstore[:foo] = "bar"
+          flag = true
+          sleep 1
+        end
+      end
+      until flag; end
+      pstore.transaction { pstore[:foo] == "bar" }
+      File.unlink("pstore.tmp2.#{Process.pid}") rescue nil
+    end
+  end
 end