PURPOSE: To increase the durability of the rotary compressor and to simplify its structure, by disposing a rotor eccentrically in a casing, fitting balllike pistons in the rotor in freely movable manner, and communicating an outlet port at the end period of rotation of the rotor.

CONSTITUTION: The rotor 5 is disposed eccentrically in the casing 1, and cylinders 11 are formed radially in the rotor 5. In each of these cylinders 11, there is fitted the ball-like piston 12 in the manner that it can be moved freely in the cylinder. Here, the outlet port 15 is formed on the inside of casing 1 where the bottom of the other open end 11b of the cylinder is passed, in particular, at a section between points D and A corresponding to the end period of rotation of the rotor 5 during the latter half cycle passing from point C via point D to point A. Since, with such an arrangement, the section of balllike piston 12 where it is held in sliding contact with the inner surface of casing 1 is changed with its rotation, it is enabled to prevent local wear of the pistons and hence to increase their durability. Further, the structure of the compressor can be simplified since an exhaust valve can be omitted.