A heat source unit control system, comprising: a heat-transfer medium circulation circuit in which a heat-transfer medium is circulable, the heat-transfer medium circulation circuit including: a tank heat exchanger that is configured to receive the heat-transfer medium, and is configured to exchange heat between the heat-transfer medium and water stored in a tank; a heat source unit that is configured to receive the heat-transfer medium and is configured to perform one of heating and cooling the heat-transfer medium, a forward channel through which the heat-transfer medium flows from the heat source unit to the tank heat exchanger; a return channel through which the heat-transfer medium flows from the tank heat exchanger to the heat source unit; a tank-water temperature sensor (S(t)) that is configured to detect a temperature Tt of the water stored in the tank; a heat-transfer medium temperature sensor that is configured to detect a temperature Tm(in) of the heat-transfer medium returning back to the heat source unit through the return channel in the heat-transfer medium circulation circuit, an additional heat-transfer medium temperature sensor (S(out)) that is configured to detect a temperature Tm(out) of the heat-transfer medium flowing out of the heat source unit through the forward channel in the heat-transfer medium circulation circuit, and a controller that is configured to control the heat source unit based on a temperature difference between the temperature Tt of the water detected by the tank-water temperature sensor and a mean value of the temperature Tm(in) of the heat-transfer medium detected by the heat transfer medium temperature sensor and the temperature Tm(out) of the heat-transfer medium detected by the additional heat-transfer medium temperature sensor. a tank heat exchanger that is configured to receive the heat-transfer medium, and is configured to exchange heat between the heat-transfer medium and water stored in a tank; a heat source unit that is configured to receive the heat-transfer medium and is configured to perform one of heating and cooling the heat-transfer medium, a forward channel through which the heat-transfer medium flows from the heat source unit to the tank heat exchanger; a return channel through which the heat-transfer medium flows from the tank heat exchanger to the heat source unit; a tank-water temperature sensor (S(t)) that is configured to detect a temperature Tt of the water stored in the tank; a heat-transfer medium temperature sensor that is configured to detect a temperature Tm(in) of the heat-transfer medium returning back to the heat source unit through the return channel in the heat-transfer medium circulation circuit, an additional heat-transfer medium temperature sensor (S(out)) that is configured to detect a temperature Tm(out) of the heat-transfer medium flowing out of the heat source unit through the forward channel in the heat-transfer medium circulation circuit, and a controller that is configured to control the heat source unit based on a temperature difference between the temperature Tt of the water detected by the tank-water temperature sensor and a mean value of the temperature Tm(in) of the heat-transfer medium detected by the heat transfer medium temperature sensor and the temperature Tm(out) of the heat-transfer medium detected by the additional heat-transfer medium temperature sensor.