To efficiently discharge heat from circulating water in a vacuum cooling device having a cooling tower.

A circulating water temperature sensor 11 for detecting the temperature of the circulating water, and an outdoor-air temperature sensor 12 for detecting the temperature of outdoor air are connected to an operation- controlling device 7. In the operation-controlling device 7, specific temperature difference Δt and specific temperature T are set, and a circulation pump 8 for feeding the circulating water to a cooling tower 4 is operated when the temperature of the circulating water being detected by the circulating water temperature sensor 11 exceeds a value that is higher than outdoor-air temperature that is detected by the outdoor-air temperature sensor 12 by the specific temperature difference Δt. When the temperature of the circulating water is lower than the value that is higher than the outdoor-air temperature by the specific temperature difference Δt, the circulation pump 8 is not operated, and water supply to a circulation water tank 3 is controlled when the temperature of the circulating water becomes higher than the specific temperature T.