PURPOSE: To restrict fluid back to compressor means just after completion of load to be cooled without using an accumulator by controlling the opening of bypass valve means in response to the judgement of judgement means after closing auxiliary valve means.

CONSTITUTION: While cooling means R cools a load to be cooled in response to the vaporization of an inflow conversion refrigerant under the compressor action of compressor means 90 and condensation action of condenser means 110, once detector means detects completion of cooling for the load to be cooled, auxiliary control means closes auxiliary valve means 140 by the control thereby. For this, the inflow of the conversion refrigerant into the cooling means is prohibited by the auxiliary valve means 140. Once the completion of the vaporization of the liquefied refrigerant is judged by judgement means, control means opens bypass means by the control thereby. Hereby, fluid back to the compressor means can securely be prevented without using additional elements such as an accumulator.