PURPOSE: To promote cooling at the time of cool-down. and to avoid interruption by the overload of an ambient temperature, etc., by making the operating differential pressure of a variable differential pressure regulating valve as a bypass valve at the time of cool-down larger than that at the time of stationary operation and reducing the operating differential pressure in inversely proportional to the ambient temperature, etc.

CONSTITUTION: A cryogenic cooling section 200 is supplied with a refrigerant body pressed by a cryogenic refrigerator 500 and a compression section 100 from a forward pipe 21, and the refrigerant body is discharged to a return pipe 25 and circulated to the compression section 100. The bypass valve 32 of a bypass pipe 31 communicating the forward pipe 21 and the return pipe 25 is opened in response to required differential pressure at that time. In said constitution, a variable differential pressure regulating valve 321 is used as the bypass valve 32. Each pressure on both sides before and behind the differential pressure regulating valve 32 is detected by each pressure detector 33, 34 respectively. An ambient temperature, etc., are detected by a room-temperature detector 102. The operating pressure of the variable differential pressure regulating valve 32 at the time of cool-down is made larger than that at the time of stationary operation on the basis of each detecting signal and reduced in inversely proportional to the ambient temperature, etc., in a control section 101.