PURPOSE: To ensure stable refrigerating cycle performance irrespective of a change in a refrigerating load in the refrigerating cycle using a non-azeotropic mixture by detecting saturated gas temperature of the non-azeotropic mixed refrigerant and a refrigerant composition at pressures on low and high pressure sides.

CONSTITUTION: High temperature high pressure refrigerant gas discharged from a compressor 1 is cooled, condensed and liquefied through a condenser 2, and in its pressure is reduced through an expansion valve 3 and is evaporated by an evaporator 4, and is thereafter sucked into the compressor 1. In this case, a saturated gas temperature/refrigerant composition detector 5 is connected through an opening/closing valve 6 with a fluid piping for connecting between the condenser 2 and the expansion valve 3 and is further connected with a suction piping for connecting between the evaporator 4 and the compressor 1. The detector 5 comprises a controller 11 including a driving circuit for controlling a cycle on the basis of output signals from a refrigerant holding pipe 7, a fluid refrigerant heater 8, a refrigerant temperature detector 10. Hereby, the flow rate of the refrigerant is controlled such that the degree of heating of the refritgerant is kept unchanged.