Japan bans Muslims? Hardly.
Source: Wikipedia:
According to japanfocus.org, as of 2009 there were 30 to 40 single-story mosques in Japan plus another 100 or more apartment rooms set aside for prayers in the absence of more suitable facilities. 90% of these mosques use the 2nd floor for religious activities and the first floor as a halal shop (Imported food; mainly from Indonesia and Malaysia), due to financial problems, as membership is too low to cover the expenses. Most of these Mosques have only a capacity of 30 to 50 people.List of mosques in Japan
You’re right. They don’t ban them - but they obviously don’t encourage them.
What I was thinking of was something I’d read that muslims cannot become Japanese citizens - but, upon looking it up again to find the source of that, it seems it’s not the case.
It looks like there’s something like 100,000 muslims in Japan (which has 126 million people).