PURPOSE: To start promptly a refrigerating device for cold temperature service where mixed refrigerant is used, and reduce swiftly the temperature continuously until a specified temperature is obtained from the beginning of operation by installing a bypass pipe which communicates with a low pressure section immediately before a vaporizer from a high pressure section immediately before other decompression devices excluding the decompression device in the final stage by way of an ON/OFF valve, and a decompression mechanism.

CONSTITUTION: When an attempt is made to open an ON/OFF valve 12 at the same time when operation starts, a part of R13 liquefied in a first vapor- liquid separator 3 flows into a vaporizer 9 by way of a decompression device 13 of a bypass pipe 14. As a result, low temperature liquefied refrigerant flows into the vaporizer, which makes it possible to generate a cooling capacity. Furthermore, the refrigerant which has left the vaporizer 9 is absorbed into a compressor 1 passing through a second intermediate heat exchanger 7 and a first intermediate heat exchanger 4. It is, therefore, possible to cool the heat exchangers swiftly.