Q: How to buy japan-music in jp-store but i still in thai store?

You can't, you can only buy from your own country's store - to be able to buy from the Japanese iTunes store you will need a valid billing address in Japan and be in Japan when buying from it. You can try requesting that the music that you want to buy be added to the Thai store, but utlimately it's the record company that will have to gant Apple a license to sell it there : http://www.apple.com/feedback/itunes.html