To provide a preserved operating method of bath unit in which a user can notice the fact that the a person trying to preserve bath boil-up operation has missed to set a preserved operation switch correctly at ON state before being too late.

A boil-up start time is operated such that boil-up of bath is completed at a set boil-up time by setting a boil-up time and setting a preserved operation switch 56 of automatic boil-up at ON state so that automatic boil-up operation is started when the boil-up start time is reached. In such a preserved operating method of bath unit, if the preserved operation switch 56 is not set at ON state although the boil-up time is set, the user is notified to this effect when the boil-up start time is reached.