Tattoo is considered that you are a part of gang organization (or Yakuza) in Japan. There for many establishments (mainly public baths, sauna and Onsen) prohibits people with tattoo from entering. It includes one point tattoo like a little butterfly and flower and even a name except for medical reasons.
If you have one point tattoo, we suggest you buy a sticker to cover your tattoo. You can question like if your culture encourage people to have tattoo, in case of like Polynesian Tattoo. I am sure there will be a lot of talk about this issue as Japan is getting ready for 2020 Tokyo Olympics.