When you say study law, do you mean a JD or LLB? I don't know Japanese University offer JD or LLB or their own legal program, Japan is a civil a law country, so it's legal system is very different from that of Commonwealth country and the USA.
No university in Japan can compete with Harvard, Yale etc in terms of reputation, employability and brag-ability, not even close, especially law school...like seriously...no comparison.
Why would you want to study law in a Asian country?? American Law school should be your first choice, followed by British law school then Australian law school. I would never study law in a non-English speaking country.
Also Japanese MBA programs are (very) crap even among Asian/Australian MBA. Insead (Singapore), HKUST (China) and Ceibs (China) are top Asian MBA programs ranked among the best in the world and no Japanese MBA program is even in the top 100 (ft ranking). Probably why Japanese economy has been in toilet for the past 20 years.
You can get JD/MBA combined from Melbourne Uni in just three years, where as typical American JD is three years. And Melbourne Law School is ranked no 5 in the world by QS. The only two American Law is better than Melbourne, that's Harvard and Yale.
So in conclusion, if you want to study Business or Law, avoid Japan...