Here is a list of frogs in Japan. First we'll need to figure out his species. He's cute. Looks a bit like an African bullfrog, though I don't think that'd be likely .

Once you know his species, we'll know his housing needs. You can make him a home that mimics the area you found him in.

You will need: a water bowl, water dechlorinator (found in the aquarium or reptile sections of a pet store), a misting sprayer, calcium with vitamin D 3 reptile or amphibian supplement, mutivitamin reptile or amphibian supplement powder, a temperature gauge and a humidity gauge (hygrometer), food (small crickets no larger than the distance between his eyes, or fruit flies if he's really small), he may need a heat source such as a reptile heat pad for the side or a lamp heater.