The master's (university, pre-doctoral) course in Japan is completed (graduated) in 2 years at the earliest and 5 years at the latest. You can graduate after completing the credits, but you can't get a diploma. You can't get a degree until your master's thesis is published.

In Japanese universities, diploma and degree certificate are integrated.

Generally, you need to complete 64 credits (depending on the university) to graduate, of which 20-26 credits are graduation thesis. In other words, if the graduation thesis is not published, it is generally impossible to get a diploma.

Many people, especially doctoral students, can't graduate because their papers can't pass (the professors don't approve them). A master's degree may take two years at the earliest, but generally it can't be faster.