早稲田 is "preferably 3 years or equivalent experience"
http://www.waseda.jp/wbs/mba_fulltime/admission.html

慶応 doesn't have any prerequisite in terms of work experience.
KBS　MBA Application - Keio Business School

They are the top 2 private universities.

早稲田 offers a course taught in English. I don't know if it's worth going there just to take English classes though. And you'll probably need to study about 2000 hours to be admitted to the course taught in Japanese. If I were you, I'd find a collage that offers exchange program with Japanese universities in the states and study Japanese before going there. Japanese is a lot harder than French or any European language, it takes 4 times the time you would spend on French to get to a certain skill level. Going to a language school doesn't cut it unless you are planning to spend 2 to 3 years full time there.

Edit: The word MBA instantly made me assume you lived in the states. What's your mother tongue? You'd need a lot less if you speak, say Chinese or Korean. Expect to get JLPT1 though, which would take about 900 hours for a Chinese guy.

Uhh.... no I'm in Thailand

The point is my father want to sent me somewhere on the planet that doesn't use English so I could have 3rd language skill. I like to study Japanese, but I know I won't have enough fluency to take Japanese course, so I plan to take English course in Japan so I would have a little bit more chance to graduate, and at the same time have more chance to practice Japanese there too.

The last thing is I don't care about where I take course as long as

1. I have chances to graduate
2. It's in Japan

Being a Thai native should still be an advantage over English natives, but I don't have the actual data. I would assume you'd be something like Chinese natives except that you'll need to memorize 2000+ Kanjis, which shouldn't be THAT hard if you are serious about it. 10 Kanjis a day, probably something like an hour a day, and it doesn't take a year.

To get into English courses, you won't need Japanese skills but you will probably need GMAT and TOEFL score. But I guarantee you, you'll acquire little Japanese skills in 2 years if you take classes in English. Japanese MBA isn't highly praised, even in Japan, so I would rather go to a collage in the states and do exchange program than taking a course in English in Japan.

JLPT 1級(now N1) is a prerequisite for many Japanese MBA courses. You'll probably need a year or more of study for that, so how about setting that as your first objective? Japanese collages are known to be harder to get in and easier to graduate, so I don't think you need to worry about graduating too much.

Thank you for your advice. May be like you said, to go to the US and admit to the college that has exchange program might be better and easier. It's not that I wanna give up Japanese but when I consider about passing JLPT1, it makes me almost cry because it will take me not less than 2-3 years to go to that point (I haven't take a test yet, but I assume that now I'm around JLPT4 level, so distant haha).