PROBLEM TO BE SOLVED: To reduce power consumption at a chiller during heat accumulation by providing a pipe through which outlet gas from a compressor is partly sent to a preheater for heating a solution in a concentrator/dilutor and is returned to an air heat exchanger. SOLUTION: In order to utilize heat which has been wasted to the external air through an air heat exchanger 2, outlet gas from a compressor 1 is allowed to flow into a preheater 26 by varying valve change-over conditions while a chiller 28 is being kept in a heat radiation state. Valves 18, 19, 20 and 27 are fully opened, and a valve 21 is fully closed. By this method, heat absorbed from cold water by an evaporator 3 is transmitted to a solution 25 by the preheater 26 and then wasted to the external air through the air heat exchanger 2, whereby heat can be effectively utilized. Further, since the solution 25 is preheated with residual heat before heat accumulation, reduction in power consumption at the chiller 28 for replenishing the heat capacity content of the solution 25 at the time of heat accumulation becomes possible.