To prevent a marked shortage of a rate of gas delivered from a bottom stage compression element from generating from a compression capacity of a high stage compression element by providing with a capacity control means for controlling a working capacity of a compression chamber so as to reduce an intake body capacity of the bottom stage compression element.

An inside of a cylinder of a bottom stage compression element 7 and an upstream side of an accumulator 2 are communicated with each other through a control circuit 99a, by a switch valve 98a before starting compression. When rotation of a driving shaft is accelerated and driven, refrigerant gas is sucked from the accumulator 2 into a suction chamber by the bottom stage compression element 7. As a result, a suction chamber capacity is increased, and also compression of the compression chamber is also advanced. Since an intake bypass passage 94 is opened, refrigerant gas is circulated to the accumulator 2. Substantial compression action is started after the piston of the bottom stage compression element 7 is passed a bypass, and however, only a part of refrigerant gas is compressed by function of a bypass hole. It is thus possible to decrease compression input of the bottom stage compression element 7 so as to start a compressor smoothly.