PURPOSE: To reduce the fluctuation of the pressure on the low pressure side by making the diameter of the intermediate section of a suction tube larger than that of both ends and making the volume of the intermediate section specific times or more as large as that of a compression chamber.

CONSTITUTION: Gas is sucked into a compression chamber 8 through a suction tube 13 by the eccentric rotation of a roller 11 during the operation of a compressor. The flow speed of the sucked gas is not constant and is accelerated or decelerated according to the position of the roller 11, if the diameter of the intermediate section 13a of the suction tube 13 is made larger than that of both ends and its inner volume is made 0.8 times as large as that of the compression chamber 8, its flow can be averaged. The pressure fluctuation on the low pressure side is thereby decreased, and the noise generated by the vibration of the valve of a check valve and the noise generated by the vibration of a pipeline or the like can be reduced.