PURPOSE: To reduce fluctuation of a discharge rate and a driving torque, and improve performance by housing a rotational body in a structural body having three curves surfaces each having a sectional area of cycloid or resembling it, and executing three compression processes by one rotation of a motor.

CONSTITUTION: One end of a rotor 2 is located at a top dead center which is an opening 4a. When a rotary casing is rotated leftward by 60 degrees by means of a motor from this position, a center of the rotor 2 is also rotated in the same direction. The rotor 2 is rotated rightward while being in contact with three curved surfaces Sa to Sc at three points. The rotor 2 is rotated in the direction opposite to the rotational direction of its center by half of the rotational angle. Valves 5a, 5b and a discharge valve 6c are opened, and intake/compression processes are executed. When the center of the rotor is rotated by 90 degrees, and the rotor is rotated by 45 degrees, compression starts in a chamber 6b. When the center of the rotor 2a is rotated by 120 degrees and the rotor 2 is rotated by 60 degrees, compression is terminated in the chamber 6c.