PURPOSE: To improve efficiency and reduce the operation rate of a compressor to the min. by calculating a set temp. according to the outside air temp. and the input to a set temp. switch and stopping the compressor when the exit temp. of an evaporator becomes lower than the set temp., thus eliminating the influence of the outside air temp.

CONSTITUTION: The outside air temp. 16 is input into a set temp. calculation part 12 through a detection part 11. While, the exit temp. 17 of an evaporator is input into a judging part 14 through a detection part 13. Further, the operation mode signal of a set temp. switch 18 is input into the set temp. calculation part 12. Said calculation part 12 determines a set temp. according to the outside air temp. 16 and the signal of the set temp. switch 18, and sends the set temp. into the judging part 14. The judging part 14 compares the set temp. with the exit temp. 17 of the evaporator, and when the exit temp. 17 lowers below the set temp., an electromagnetic clutch 19 is operated to stop a compressor. Therefore, the efficient cooling efficiency can be secured for the variation of the outside air temp., and the operation rate of the compressor can be reduced to the min.