Yes, it is quite safe to drink tap water in Japan.

Tokyo Waterworks writes on their homepage that the tap water is completely safe to drink and has a soft taste. Technically the water is soft and is that gives a strange taste for me who is used to hard water.

If you don’t like the taste or need water when outside, then every convenience store sells a big water bottle for ~100 yen (7-10 NOK).

A lot of people install water filters at home, but that might be due to historical reasons. In the 1960s era, there was a strong smell from the water due to pollution. At that time it was a widespread perception that water in metropolitan areas smells bad.

But a lot has changed since that time. In 2015 Tokyo Metropolitan Government Bureau of Waterworks held a series of public events where they conducted blinded water experiments. People were asked to taste water from bottled water and tap water. But the participants did not know the source of the two water samples. 46% of the 50 000 participants preferred tap water.

Personally, I prefer bottled water, but I will most likely try to get used to tap water to save money and the environment.