A heat exchanger (1) on the secondary heat source, which exchanges heat with a heat exchanger (12) on the primary heat source in a primary cooling circuit (A), is connected with an indoor heat exchanger (3) through a gas pipe (6) and a liquid pipe (7). A tank (T) storing a liquid cooling medium is connected at its lower end to the liquid pipe (7) and at its upper and to a pressure adjustment mechanism (18). Check valves (CV1 and CV2) are disposed on both sides of the connecting portion of the tank (T) with respect to the liquid pipe (7). The internal pressure of the tank (T) is changed over alternately between a high pressure state and a low pressure state by the pressure adjustment mechanism (18) so that the liquid cooling medium is supplied to the indoor heat exchanger (3) at the time of the high pressure operation, and the liquid cooling medium is recovered from the heat exchanger (1) on the secondary side to the tank (T) and is circulated by a secondary cooling circuit (B) at the time of the low pressure operation.
