A combined air-conditioning and hot water supply system simultaneously executes the cooling operation of a use unit and the hot water supply operation of a hot water supply unit, wherein the combined air-conditioning and hot water supply system operates in a cooling priority mode when the temperature differential ΔTwm between a set hot water supply temperature Twset and the inlet water temperature Twi of a plate water-heat exchanger is smaller than a priority operation determination threshold M that is set in advance, and operates in a hot water supply priority mode when the temperature differential ΔTwm becomes equal to or higher than the priority operation determination threshold M. This simultaneous execution of cooling and hot water supply operations prevents hot water from running out.