PURPOSE: To compensate for reduction in a flow rate owing to a low water pressure and to perform a drop of a high flow rate by a method wherein operation of a circulating pump is controlled based on an output from a pressure sensor arranged to a bath circulating circuit and when a pressure is below a given value, the circulating pump is operated to perform forced conveyance.

CONSTITUTION: During a drop of a full automatic bath burner, when automatic operation is started, a three-way valve 27 of a bath circulating circuit 20 is switched to the direct pressure hopper 35 side and a drop water solenoid valve 31 is opened. Hot water heated by a heat-exchanger 2 for the feed of hot water is dropped from a feed hot water passage 28 for a bath use only to a bath circulating circuit 20 for bath through a direct hopper 35. It is then detected by a pressure sensor 23 located in the bath circulating circuit 20 whether or not a detecting pressure P attains a given pressure and when it exceeds a given pressure, an automatic drop is effected by means of only a feed water pressure. Meanwhile, when it is below a given pressure, a circulating pump 26 is operated to start forced conveyance, and a large quantity of automatic drop hot water is fed. When a pressure attains a value outside a range of a specified value, the circulating pump 26 is stopped.