To remove cold water sandwich phenomena by exchanging the heat of the hot water heated with the heat exchanger of a hot water supplier with the heat of the water which enters the heat exchange, being taken into a hot supplier from a water supply port and cooling the overheated hot water to moderate it, and also, heating supply water.

A heat exchange tank 6 is of a double pipe type, and this is provided with a fin 12 around the inner tube 11, and is provided with a water supply inlet 14 at the upper part of the outer shell case 13, and is provided with a water supply outlet 15 at the lower part. This heat exchange tank 6 exchanges heat between supply water and the delivery water, and the temperature of delivery hot water is lowered, and the temperature of the supply water goes up. In this case, the quantity of heat conduction becomes larger, the larger the temperature difference is, and the temperature difference to be changed is large. Accordingly, the overheat of the hot water coming out of the heat exchanger 4 is moderated greatly. Moreover, even if supply water passes the heat exchanger 4 as it is in a while after a burner 5 operates, it is not kept quite cold and the cold water sandwich can be moderated because the heat is exchanged with that of overheated hot water.