PURPOSE: To prevent the occurrence of deficiency in hot water in a bath tub occasioned by back washing by a method wherein a circulation purifying heat insulator having a filtering device and a heater is provided, and when back washing of a strainer is completed through control of switching operation of the flow passage of the filtering device, control is made such that a hot water feeder is driven.

CONSTITUTION: A circulating pump 23 performs 24-hour operation and hot water in a bath tub 1 orderly circulated through a strainer 22a and a heater 21. When a time comes to 18 o'clock, a back washing command is transmitted to a filtering device 22, and the filtering device 22 is switched from a washing state to a back washing state. At a point of time when an amount of a back washing water amounts for 50l, the filtering device is returned to a washing state. At the point of time when back washing is completed, a detected water level by a water level sensor LS is below a specified water level, whereby an automatic hot water feeder 3 is driven. The bath tub 1 is full of hot water and brought into a state that bathing is practicable. Each time, after completion of back washing, hot water in the bath tub 1 is consumed owing to bathing during a time to 7 o'clock, the automatic hot water feeder 3 is driven and an amount of hot water in the bath tub 1 is held at a constant value.