[tommasi]

First of all, the best business school in Japan is probably MBA program at Tokyo University. Many other well know universities offer MBA but Tokyo University is clearly the solid choice.

In terms of Japanese skills required to attend Japanese universities, I strongly believe you need to be close to fluent and it takes about maybe 4 years minimum to reach that level if you really study hard. Most international students have been studying Japanese for years so it'll be hard if your Japanese is weak.

So overall, do not do your master's degree in Japan because MBA is not so established in Japan and you need to be fluent in Japanese even before attending.