“In Japan tattoos have long been stigmatized for their association with organized crime gangs, the Yakuza, who pledge their allegiance with full-body markings. Consequently, anyone with ink — regardless of their profession — cannot usually use public swimming pools, hot springs, beaches and even some gyms.”
This situation might change in the future. However, please be aware of tattoo rules at gyms in Japan.
For instance, Gold’s Gym Japan has strict rules about tattoos. If your tattoos were bigger than 15cm x 15 cm (A palm size), you
are not allowed to go into gym even if you cover up. It is because your tattoos will be exposed at lockers rooms.
Japanese people know that travelers are not one of those gangs, however, if gym let you in, there is a chance that those gang get angry and might attack gyms.