Embassy in US says the passport must be "valid for period of stay":
Q I would like to visit Japan for one week in August as a tourist but my passport is valid until October. Does my passport have to be valid for at least six months?
A Any foreign visitor who wishes to enter Japan must have a passport, which will remain valid during the period of stay. However, if you have a U.S. Refugee travel document or a U.S.re-entry permit (permit to re-enter), we require 6 months of validity. If you have a temporary passport, emergency passport or other types of travel documents, please contact us in advance.
https://www.sf.us.emb-japan.go.jp/it...m02_01_08.html