To economically increase the heat utilization efficiency of an entire hot water supply system, and to enable quick hot water supply while suppressing the lowering of a temperature.

When hot water in a hot water storage tank 22 is supplied to an auxiliary hot water storage tank 54 via a hot water feed pipe 2 and a first passage in a passage selector valve 52, if a temperature detected by a temperature sensor 53 is lower than a predetermined temperature, a water feed stop valve 51 is closed and the hot water in the hot water storage tank 22 is delivered to a water feed pipe 5 via the hot water feed pipe 2, a second passage in the passage selector valve 52 and a connecting pipe 56 until the detected temperature exceeds the predetermined temperature.