PURPOSE: To contrive the suppression of vibration and noise and the improvement of durability by fitting a ring, provided with a polygonal peripheral part corresponding to a number of pistons, to the periphery of an eccentric cam while forming a cutout, connecting a suction port of the piston to communicate with a cam chamber, in the periphery of the ring.

CONSTITUTION: When a rotary shaft 2 is rotated, an eccentric am 3, connected to this rotary shaft 2, is integrally rotated. While a ring 10, fitted to the outside of the eccentric cam 3, is not rotated on the own axis while rotating the eccentric cam 3 in the internal peripheral part 11 of the ring 10, and it is guided by the eccentric cam 3 and displaced along a circular locus with an eccentric distance of the eccentric cam 3 serving as the radius. When the eccentric cam 3 performs one rotation, a peripheral part 12 of the ring 10, while changing a position of a cutout 14 for a suction port 13 of a piston 6, performs one reciprocation laterally while vertically, driving the piston 6. As the result, the piston 6 advances into an oil chamber 5 while the cutout 4 is detached from the suction port 13, disconnecting the communication of the oil chamber 5 with a cam chamber, thus operating oil is compressed.