Aisome-no-yu
別所温泉 相染閣 あいそめの湯
No restriction. The bath’s own FAQ puts the question plainly, whether you can bathe with a tattoo or irezumi, and answers that you can. No seal, size limit or declaration is mentioned. This is the one bath in Bessho with a published answer.
タトゥーや刺青があっても入浴できますか? ご入浴いただけます。
Best for: A long soak with a view over Ueda straight off the train, with nothing to cover
Built by Ueda City as a municipal bath in 1972 and rebuilt on the present site in 2008. Indoor and open-air baths, simple sulfur water, 20 washing spots on each side with body soap and rinse-in shampoo. 10:00–22:00, last entry 21:20; closed the 2nd and 4th Monday (the next day when that Monday is a holiday). Adults 650 yen, school-age children 300. Rock sauna (ganbanyoku) 650 yen extra with wear and towels. Towels are sold, not free. No credit cards; PayPay accepted. Food brought in is not allowed; the restaurant is Biranju. Adults wearing diapers may not bathe. The page is hosted on the Bessho Onsen tourism association’s site.