I know this is an old thread but was browsing and wanted to throw in my 2 cents.
As ihatefall said above, I also highly, highly recommend that if you stay in Japan you try to get into a foreign company rather than a Japanese one. I know some Japanese companies are changing the way they work, like literally pushing people out the door at 6:00 on "no overtime" days, but if you are from a Western culture you are going to have a major problem trying to be a Japanese office worker in a traditional Japanese corporate environment.
Also, I was naive enough to expect that the Japanese work culture wouldn't be so strong in the US branches of a Japanese company but I was wrong. Most of the Japanese staff will continue to work as if they are still in Japan. American staff will generally not play the salaryman game, but then in the end you are back in an ALT-like situation where you feel like you are somehow being a bad employee by going home on time. Whatever your field I urge you to seriously consider if you want to work for a Japanese company.