PURPOSE: To make the pitch and level of discharge pulsation reducible, by dividing each vane of paired rotors into two parts in the axial direction, and partitioning off the interval with a partition plate, while shifting each rotational phase of these vanes on both sides of the partition plate to some extent mutu ally.

CONSTITUTION: Two vanes of rotors 5 and 6 are constituted of divided vanes 5a, 5b, 6a and 6b halved in the turning shaft direction of these rotors 5 and 6, and a partition plate 7 supported by a casing 1 is set up between these divided vanes 5a, 5b and 6a, 6b. These vanes 5a, 6a and 5b, 6b engaged with one another are supported on a turning shaft so as to make each differ as far as 90 degrees in rotational phase, so that phase, where pulsating pressure of the gas discharged out of both sides of the partition plate 7 is varied, is shifted to some extent, and since variations in the pulsating pressure interfere with each other, the pitch and the level are halved.