PURPOSE: To permit to maintain the gas temperature of a pressure equalizing chamber below a predetermined temperature by supplying a part of the gas of an intermediate stage, which is under compression stroke, into the pressure equalizing chamber of delivery side.

CONSTITUTION: A part of the gas under the compression stroke is taken out of the intermediate stage C of a compression unit 1 and is supplied into the secondary pressure equalizing chamber 7 of delivery side after being chocked by an orifice 15. The temperature of the gas in the secondary pressure equalizing chamber 7 or the pressure equalizing chamber 3 of suction side, which is communicated with the chamber 7, may be maintained below the predetermined temperature by a method wherein the amount of the compressed gas, mixed into compression gas, is regulated by the orifice 15. According to this method, oil mist, filling each neighboring sour oil chambers 8, 4, is not heated to a temperature higher than a predetermined value, and oil film sealing effect in each sealing chambers 9, 5 may be prevented from deterioration.