Japan’s tap water is 100% safe to drink. It even tastes nice, which may be surprising for a large metropolis such as Tokyo.
That said, I buy bottled for the first week I’m in the country. That lets my stomach adjust to a new microbial environment more smoothly.
I find that just about anyone can have their stomach get mildly upset when visiting a new country – even friends who consider themselves as having “iron stomachs” at home. Others are perfectly fine.
In short, it seems like luck of the draw dictates who might get an upset tummy.
After a week, I feel more comfortable going straight to the tap at the hotel.
As of summer 2019, it’ll be easier to head to the tap. Tokyo is installing public water fountains that draw from the tap supply. It fits a few purposes, including letting the public know that the water is safe to drink.
The public fountains solve more than that. Additionally, they encourage the use of reusable water bottles, rather than single-use plastic. This is a very important step in a country dealing with a plastic waste problem.
Further, with worryingly high temperatures in the summer just before the Tokyo 2020 Olympics, the public fountains aim to lower the risk of heatstroke among an expected flood of visitors.
So, after being on the ground a few days, you can switch to Japan’s tap water. You’ll be just fine.