Disclosed is a refrigerant cooler which can cool circulating refrigerant down to a predetermined temperature efficiently under stabilized state even if the cooling temperature of the circulating refrigerant is near the freezing point thereof. The refrigerant cooler cools down circulating refrigerant for cooling an object to be cooled such as a cryogenic reaction tank (11) by performing indirect counterflow heat exchange of the circulating refrigerant and cryogenic liquefied gas by means of a heat exchanger (17). A first flow control means (23) for controlling cryogenic liquefied gas supply by detecting the temperature of circulating refrigerant led out from the heat exchanger (17) by a first temperature detection means (20), and a second flow control means (25) for controlling cryogenic liquefied gas supply by detecting the temperature of a heating surface in the heat exchanger by a second temperature detection means (21) are arranged in series in a cryogenic liquefied gas introduction passage (16) for supplying cryogenic liquefied gas to the heat exchanger.