Yes, tap water is drinkable in Japan.
This also goes for brushing teeth and little kids accidentally drinking bath water, etc.
It is also more than fine and safe for boiling for making baby formula. Just as an FYI, the word for baby formula in Japanese is 粉乳児用のミルク (nyujiyou no kona miruku). See here for my tips on finding baby food in Japan. Some drugstores and supermarkets sell something called “Baby Water” which is sold as an option for families making formula when they don’t have the option to boil the water first – I don’t know too many people who use this – it’s clever marketing though.
In Japanese apartments we have always used a water filter and when in hotels in the big cities I prefer to drink bottled water, personally – but that is a taste thing – not a safety concern.
I drink the complimentary water provided in jugs at restaurants in Japan. Japanese tap water is safe to drink.
If you are looking for places to fill up your water bottle while in Japan – download the MyMizu app. See here for my list of all of my recommended apps for travelling in Japan as a family.
See here for my tips on buying distilled water in Japan for a C-PAP machine.
