PURPOSE: To restrain the abnormal rise in an intermediate pressure, and to prevent the damage or like in a valve mechanism and bearing parts, by carrying out a load control operation only for a given time after starting, so that respective numbers of load cylinders on low-stage side and high-stage side have a specific relationship.

CONSTITUTION: A timer 19 is connected in series with the solenoid valve 15a on one side on low-stage side and the solenoid valve 15c on high-stage side. Denoting by ZLO the load cylinder number on the low-stage side due to the low-stage side operation cylinder and low-stage side control cylinder, and by ZHO the load cylinder number on the high-stage side due to the high-stage side operation cylinder and high-stage side control cylinder, the timing for controlling load is set in a timer 19 so that the ratio of ZLO to ZHO is to be 2 to 2. And to the time 19, a timing contact 20 which is connected in series with a solenoid valve 15b on the other side on the low-stage side is provided, and this is opened only at the set timing of the timer 19. Thus, in the set timing after a compressor has been started, the load control operation is carried out so that the relationship ZLO/ZHO<2 can be kept, enabling the abnormal rise in an intermediate pressure to be restrained.