A control apparatus for a heat pump hot water supply system of the indirect heating method is provided such that a change in heat transfer efficiency of a second heat exchanger is estimated by using either or both of a tank water temperature and a temperature of a second refrigerant flowing through a water circuit. When the heat transfer efficiency is determined to be high, an operation switching unit raises an output of a heat source device. When the heat transfer efficiency is determined to be low, the operation switching unit lowers the output of the heat source device.