diff --git lib/pstore.rb lib/pstore.rb
index 65cce7b..4d80e2c 100644
--- lib/pstore.rb
+++ lib/pstore.rb
@@ -318,8 +318,8 @@ class PStore
   #
   def transaction(read_only = false, &block)  # :yields:  pstore
     value = nil
-    raise PStore::Error, "nested transaction" if @transaction
     @lock.synchronize do
+      raise PStore::Error, "nested transaction" if @transaction
       @rdonly = read_only
       @transaction = true
       @abort = false
diff --git test/test_pstore.rb test/test_pstore.rb
index e10ce90..32d79ea 100644
--- test/test_pstore.rb
+++ 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