This sign on a sento (public bath house) says “Irezumi no kata okotowari shimasu.” (People with tatoos are not permitted).
A lot of non-Japanese people with tattoos worry about whether they will be allowed into sento or hot springs, but it’s not usually a problem. There is a distinction in Japan between “irezumi,” Japanese-style tattoos done with traditional methods, and “tatu,” modern tattoos done with electric tattoo machines. As long as a person isn’t covered with them, there shouldn’t be a problem because the “no tattoos” rule seems to be just a polite way to say “no yakuza.”
Here’s a really interesting thesis written by a woman who studied traditional tattoing in Japan. At first she tried to visit tattoo studios and talk with yaks, but she recounts how she had more success working in a hostess club and getting gangsters to talk about and show off their tattoos there! It’s pretty long and academic, but full of interesting information: http://www.nootrope.net/koi/tattooing_in_japan.html
If you’d like a shorter, easier read, try: http://www.vanishingtattoo.com/tattoo_museum/chinese_japanese_tattoos.html