To save energy by stabilizing cooling water temperature when operating a refrigerating machine.

When the refrigerating machine 10 is operated, measurement results determined by measuring dry-bulb temperature and relative humidity and computing wet-bulb temperature, a maximum capacity of a cooling tower 14, a capacity of the cooling tower during partial load operation and coolable temperature determined by using the respective capacity data are determined. When the refrigerating machine requiring cooling water is operated, rotational frequency of a cooling fan motor 26 of the cooling tower is continuously controlled, and the operation of the refrigerating machine is stabilized to reduce a range of fluctuation of the control by stabilizing the temperature of the circulating cooling water, so as to contribute to energy saving operation.