a japanese bathtub is easy to maintain: all it needs is 1 minute of weekly care and plenty of affection...!

Hinoki wood has a unique natural protection against rot, mold and fungi, but you should use some common sense. If the bathroom is not ventilated or if you leave the bathtub full of cold water on a daily base, some stains may appear overtime. No need to worry. Just pour abundant warm water on the area until well absorbed by the wood. Then wipe gently with a cloth sprayed with alcohol for disinfection. The stain will disappear completely.

ADDED 2009.04.22
For toughter stains you can use -wisely- also some bleach for clothes. Wash abundantly with water after application.