PURPOSE: To cool water in a water tank near a freezing temperature by setting the velocity of water running in a heat exchanger to a relatively high level and halting the flow of refrigerant when a water flow side-temperature in the heat exchanger has reached the freezing temperature or when a differential pressure between the water in-flow side and water out-slow side of the heat exchanger has reached a specified value.

CONSTITUTION: The velocity of water running in a heat exchanger 16 is set to a relatively high level and the flow of refrigerant is halted when the temperature of water on its flow-out side of the heat exchanger 16 has reached a value around a freezing temperature of water or when a differential pressure between the flow-in side the flow-out side of water in the heat exchanger 16 has reached a specified pressure. The temperature of water manufactured is 0.1°C higher than a specified value for the freezing temperature of water. What is more, when a definite time has elapsed since the refrigerant stopped its flow, the operation is reopened by allowing the refrigerant to flow. This construction makes it possible to prevent the freezing of water in the heat exchanger even if the temperature of water approaches its freezing temperature and maintain constantly the water temperature near the freezing temperature without lowering the service life of a compressor.