PURPOSE: To ensure the easy prevention of a biological deposit with a small amount of hot water by forming an air layer via the discharge of a part of cold water after the sealing thereof in a flow passage between inlet and outlet valves, filling hot water into the air layer for forming a hot water layer and shifting the hot water layer to the lowest part of the flow passage.

CONSTITUTION: With flow passages X, Y and Z full of cold water, inlet and outlet valves 2a and 3a are totally closed and cold water in the flow passage Y is kept sealed. In this condition, cold water in a flow passage Y1 is drained with a discharge pump 26 and the flow passage Y is filled with compressed air from a pressure tank 19. Then, hot water is filled into the flow passage Y1 from a hot water reservoir 17 via the operation of a feed water pump 22. Thereafter, the discharge pump 26 is operated to discharge a certain amount of cold water deep in a hot water layer to the flow passage Z, thereby shifting the hot water layer lower. After holding the hot water layer for a certain time, the discharge pump 26 is again operated to shift the hot water layer (c) lower. The aforesaid processes are repeated until the reading of a thermometer 27 reaches the predetermined value, thereby killing an organism in the flow passage Y.