PURPOSE: To perform reliable sterilization of the interior of a bathtub and to prevent an adverse influence from being exercised on a bathing person and a circulating circuit containing a heat exchanger by a method wherein an exhaust circuit communicated to the open air is branched from the downstream side of the connection part of the ozone discharge port of the circulating circuit.

CONSTITUTION: An ozone discharge port 31 of an ozone generator 3 is connected to a circulating circuit 2. Ozone generated by the ozone generator 3 is fed in circulating water circulating through the circulating circuit 2. The ozone is carried on the flow of circulating water to the exhaust passage 7 side located on the downstream side thereof, and during the flow of it to the exhaust pas sage, germs in the circulating water are sterilized. When the ozone completing sterilization of the circulating water flows to an exhaust passage 7 part on the downstream side thereof, it is removed from the circulating water by means of the exhaust passage 7 communicated to the open air and exhaust in the open air. This constitution causes the feed of only sterilized water containing no ozone to a bathtub B and sterilizes bath water in the bathtub B as it is circulated through the circulating circuit 2.