To provide a control method for automatically recognizing the reduction of a capacity of an evaporator.

Brine flowing in a heat load apparatus F flows in a heat transfer tube DN in an evaporator 12 and flows out after the heat of evaporation is taken by a refrigerant, and performs circulation. A pressure sensor S1 measuring pressure in the evaporator and a refrigerant liquid temperature sensor S2 measuring a temperature of a refrigerant liquid accumulated in the lower part of the evaporator are provided, a memory is prepared in which a corresponding relationship of a refrigerant saturation temperature to pressure in accordance with the mixed ratio of an absorption liquid in the refrigerant liquid is previously stored, and the ratio of the absorption liquid mixed in the refrigerant is detected by reading a saturation temperature of a pure refrigerant corresponding to the pressure measured by the pressure sensor from the memory and calculating a temperature difference ΔT1 between the measured refrigerant temperature and the read saturation temperature of the pure refrigerant.