Thanks for your response.
I have changed jdk/jre from version 1.5.0_04 to 1.6.0_10.
I have reinstalled the jBPM5.1.0, and now I can see the designer.
(Before, in computer->property->environment variable, I configured java_home as version jdk 1.5.0_04, but windows cmd.exe used jre 1.6.0_10.
Now, I have changed java_home as version jdk1.6.0_10.)