PURPOSE: To prevent freezing by providing a temperature sensor and valve for opening and closing a pipe passage in a piping section to a bath tub and operating for a certain time the valve and a heat source machine by the signal of the temperature sensor.

CONSTITUTION: The water that entered a heat source machine 1 from a supply water opening 4 is heated by a burner 5 and heat exchanger 6 and supplied as hot water to a bath tub 2 via a solenoid valve 7 and through a pipe 3. When the hot water temperature is low, the low temperature is detected by a temperature sensor 8 and its signal is inputted to a control device 9 before the water in the pipe 3 freezes. The control device 9 judges freezing prevention operation by a freezing prevention operation judgement device 10 and by its signal solenoid valve drive means 11 opens the solenoid valve 7 according to a time determined beforehand and at the same time heat source drive means 12 ignites the burner 5. With this arrangement hot water heated by the heat exchanger 6 flows through the pipe 3 and heats it, and prevents its freezing.