PURPOSE: An air reservoir is provided at the top part of the sealed cycle, and also device that detects the air density from the relative gravity difference between coolant gas and air in the air reservoir is provided, and thereby the lowering of cooling efficiency to be caused by the intrusion of air is prevented.

CONSTITUTION: Air reservoir 8 is provided at the top of No. 1 header 4, which is integrated with condenser 5 via connecting pipe 10, so as to retain the air which has entered here. A relative gravity detector is also provided here. In this way, by detecting the relative gravity of the mixed gas which varies with the intrusion of air, the required measures can be taken before operation failure occurs.