My personal take on getting a masters in CS is that it isn't completely needed. With an undergrad in CS that's more than enough to get a good job in tech. I see a masters as either the start of a path into teaching (meaning a phd would follow), or one wants to get into the really competitive companies (Google, Apple, etc..., however a masters isn't really needed for those either, it's just helpful). However, if your undergrad wasn't in CS than yes, a masters would help to break into tech, but keep in mind, a boot camp and a personal project can also help one break into tech, at a fraction of the time and cost.
Regarding earning a masters in Japan in CS, I agree that it would be most useful if you stayed in Japan afterwards, cause you'd want the reputation of the program you enter to help you in your career. Tokyo U is knowns a great university in Japan, I'm not sure about its brand recognition and network internationally to your potential employers.
At the end of the day, I think you should choose a masters program based on reputation and network. Get into the best program you can afford to get into. Example: if you get a masters in CS from MIT or Stanford, I don't think any company in Japan (or anywhere else for that matter) will overlook your resume. You'll definitely get someone to take a closer look. I'm not sure you'll get that same treatment with a Japanese CS degree on your resume.
This is just my opinion, I have been wrong before