Japan is often compared to a gigantic sponge because of the thousands of thermal springs it has. Japanese have enjoyed thermal springs since two thousand years; warm baths are the most favorite places of tourist interest. Once you are in Japan, don't miss them. It will enchant and renew you for sure.
Not only the aged, but also the young ones like it in Japan, so public baths are open all year through, and every age-group love bathing. In most of the public baths the thermal spring is in a cave, there are some others where you can find basins in the open air. Public baths can be used in all seasons. Even in a light snowfall you can pleasantly relax there. 
 

 Rules for public baths:

• Men and women have separate pools, and entries
• Clothes should be taken off in the changing room, and left on shelves or in baskets
• Only soap and towel can be with you in the bath, but you must not use soap in the hot water pool
• First you have to have a thorough wash with soap in the hall, then you may enter the common pool
• It is forbidden to wear swimsuit in the bath
• It is serious indecency to dip your head in the water, or swim in the pool
Source: http://www.terebess.hu/keletkultinfo/lexikon/japanfurdo.html