Japanese encoding issue

Hi,
i have just installed WordPress in Japanese,
the displayed font are working without any problem,
but if i try to edit a post or add a news post is japanese the text change into ????? , and if i try to change the post title nothing is saved, same if i try to setup the blog tittle in japanese, nothing is saved…

but something strange, all the demo data (in Japanese) are working perfectly, the problem occur only when i try to edit or add a news post in Japanese.

Thanks you in advance, any help will be greatly appreciated.