To attain high accuracy in a control of an inlet temperature of a heat exchanger for undercooled water by a construction wherein an ice storage tank and the heat exchanger for undercooled water are connected by a cold water supply passage and a supply water passage connecting the cold water supply passage with the ice storage tank is provided with a heating means and a flow control valve opening and closing on the basis of detected values of a temperature sensor provided for the cold water supply passage.

Water to be cooled which is supplied from an ice storage tank 3 circulates through between outer and inner tubes of a heat exchanger 2 for undercooled water, and the lower part of the ice storage tank 3 and the inlet of the outer tube are connected by a cold water supply passage 10. The lower part of the ice storage tank 3 and the upstream side of a temperature sensor 12 inserted in the cold water supply passage 10 are connected by a supply water passage 16, and a flow control valve 17, a supply water pump 18 and a radiator 7 as a heating means are inserted in sequence from the downstream side of the supply water passage 16, i.e., from the cold water supply passage 10 side, while the flow control valve 17 and the temperature sensor 12 are connected by a thin pipe 19. The flow control valve 17 opens and closes on the basis of detected values of the temperature sensor 12 and the supply water pump 18 is driven by an operation signal of this valve.