PURPOSE: To downsize an absorber by decreasing the amount of a solution in the absorber, by a method wherein both a first solution tank and a second solution tank are connected to the absorber and by a control device, the solution strong in the absorber is transferred to the first solution tank and the solution weak in the second solution tank is transferred to the inside of the absorber.

CONSTITUTION: In the case where there is no load to be cooled; when a refrigerant pump 7 is stopped and a generator 2 is heated, a solution 14 in a absorber 4 is condensed, and when the concentration of the solution reaches a preset value, a detector 16 for the concentration is actuated and the heating of the generator 2 is stopped. By a control device 25, control valves 30, 32 are opened, and when a strong solution discharged from a solution pump 6 is filled up in a first solution tank 21, the control valves 30, 32 are closed. After that, when control valves 31, 33 communicating with a second solution tank 22 in which a weak solution is filled, are opened and the inside of the absorber 4 is filled up with the weak solution, the detector 16 for the concentration is actuated and a control valve 17 is opened, following which the generator 2 is heated and the solution is again condensed. On the other hand, when the load to be cooled exists without a heat source for the generator 2, a refrigerant 13 is evaporated in an evaporator 3 using the strong solution condensed in the absorber 4, and thus chilled water is produced.