When heating up the water in the lower inner part of a hot water tank by a heat pump unit, a control unit stops the heat-up operation while leaving the hot water in an amount corresponding to the residual water capacity in the hot water tank detected by water level thermistors. In the heat-up operation in the hot water tank, the residual hot water capacity in the tank is first detected, then the heat-up operation is stopped when the residual hot water reaches an amount corresponding to the detected residual hot water capacity. At this point, the hot water is stored so that the hot water newly heated up by the heat pump unit is laid above the residual hot water for the preceding day. Consequently, the reduction in the operating efficiency of the heat pump cycle is prevented without supplying the medium-temperature residual hot water to a condenser.