The present invention provides a heat source unit capable of being used in either an air conditioner for switchable cooling and heating operation or an air conditioner for simultaneous cooling and heating operation. An air conditioner principally includes one heat source unit, a plurality of utilization units, and a connecting unit provided for each utilization unit. The heat source unit uses water as the heat source, and principally includes a compressing means, a main heat exchanger, a first switching means, a main refrigerant switching means, an auxiliary heat exchanger connected in parallel with the main heat exchanger, a second switching means, an auxiliary refrigerant switching means, and a liquid receiver. The auxiliary heat exchanger is capable of switching between functioning as an evaporator and a condenser, by the second switching means.