See below for more details on drinking age and alcohol purchase age in Japan
This is not legal advice. Check with local law enforcement what is the legal drinking age BEFORE drinking or purchasing alcohol while in the country.
Details
More legal details
Click here: Wikipedia Legal Drinking Page
Purchase Age
On Premise20
Off PremiseNone
Drinking Age
Private20
PublicNone
More legal details
If the answer above is None it means there is no specific mention in the legislation, but you still need to ask local law enforcement before drinking in Japan.
You can find more legal details and links here:
Wikipedia Legal Drinking Page