According to the WHO, tap water is safe for consumption for 99% of the country.
Tap water is fine to drink in Japan; in some rural areas, locals swear by its health benefits.
According to the CDC tap water is generally safe for consumption.
According to the Numbeo water drinkability and accessibility index, tap water has a score of 70.4 / 100 (100 being best).
Numbeo considers this score to be "High"
1 adult could save about 832$ per year in Japan by drinking tap water instead of buying bottled water.
๐ณ Drinking tap water would also help with reducing your plastic footprint and improve environmental pollution.
Like in most countries, water quality and accessibility differs from location to location, therefore it's the best to check the information available on that place.