PURPOSE: To previously avoid liquid return upon restarting by providing superheat degree altering means for obtaining larger suction superheat degree than suction superheat degree at the time of full load operation, at the time of partial load operation.

CONSTITUTION: Superheat degree altering means 7 for obtaining larger suction superheat degree than that at a full load operation, at the time of partial load operation is provided. The means 7 has temperature difference correcting means for correcting temperature difference to be detected at the inlet of an evaporator 5 to a smaller value than an actual value. A switch A is opened, a switch B is closed at the time of partial load, an inlet temperature sensor T1 is inserted in series with a quasi-resistor R, its quasi-resistance value is added to the resistance value to meet the detected inlet temperature of the sensor T1 to provide a larger apparent value than the actually detected inlet temperature and to input it to a regulator 6, and the difference (T2-T1) between the detected outlet temperature by an outlet temperature sensor T2 and the inlet temperature is corrected to a smaller value than the actual value. Thus, larger suction superheat degree than that at the time of full load is obtained, and an accident due to liquid return is prevented at the time of restarting.