A hot water storage tank (3) and a circulating path (12) coupled to the hot water storage tank (3) are provided. An operation is possible wherein low-temperature water, let out from a lower part of the hot water storage tank (3) and entering the circulating path (12), is heated and boiled up by a heat pump heating source for forwarding to an upper part of the hot water storage tank (3). A refrigerant circulating circuit of the heat pump heating source is provided with a defrost circuit (38) for supplying hot gas from a compressor (25) to an air heat exchanger (28). It is possible to perform a defrost operation for providing a supply of hot gas to the air heat exchanger (28), with a water circulation pump (13) of the circulating path (12) held in abeyance. The water circulation pump (13) is made active if the defrost operation has been continued for not less than a predetermined time period since the start thereof.