PURPOSE: To eliminate the leakage of gas due to the thermal deformation of a cylinder block by a method wherein the pumping operation of each cylinder is stopped in order alternately in the same frequency to equalize the thermal loads of all cylinders.

CONSTITUTION: In the light load operation of the compressor, respective cylinders 21W26 are stopped sequentially one time in every one operation cycle of the full load operation whereby one operation cycles of respective cylinders are elongated to two times of the full load operation. The operations of respective cylinders 21W26 are stopped in the order of 25→26→21→23→22→ 24 during run ning under elongated one operation cycle and thereby effecting 50% reduced cylinder operation. Accordingly, respective cylinders stop the pumping operations so as to equalize the thermal loads of all cylinders and the leakage of gas due to the thermal deformation of the cylinder block may be eliminated.