PURPOSE: To cool in a high thermal load state at a maximum and to prevent freezing of an evaporator in a moderate or low thermal load state by rotating a compressor to a maximum limit rotation in the case of a large temperature load, and correcting a cut-off point of the pressure in the case of a low temperature load.

CONSTITUTION: A controller 50 reads a discharge pressure or a suction pressure of an evaporator 34 when a compressor clutch 14 is ON, and it starts counting of a timer time for raising a pressure by releasing the clutch 14, in the case when the suction pressure is equal to or lower than the clutch off pressure value. If the suction pressure is greater than the clutch off pressure value, the clutch 14 is let in, and a timer for raising the pressure is stopped. A timer time elapsed from the time when the discharge pressure has arrived at the clutch OFF pressure value to the time of arrival at the clutch ON pressure value is counted, and the clutch OFF pressure value is reduced according to a predetermined schedule of a change in the timer time.