Takashi,
It depends on your browser. For FireFox, go to "Tools" at the top of the page, then "Options", then lastly click on the "Privacy tab". If you are worried about ADISC cookies, they are automatically deleted when you sign out. Just click Log Out, and voila.
Normally you can find it under "Tools" in most browsers. Control + H brings up the History. I suggest clearing that too.
Done, and safe!