PURPOSE: To improve mechanical efficiency and reduce a cost by providing a rotary cam which is rotatably supported on the eccentric part of an output shaft and having three cam projections in circumferential direction, and changing the capacity of three operating chambers which are formed between the rotary cam and the cam surface so as to be generate power.

CONSTITUTION: The end parts of an output shaft 1 on which an eccentric part 2 is provided on an intermediate part are rotatably supported on a cam housing 3 by bearings 4, 5, a rotary cam 6 is rotatably supported through a bearing 7 on the eccentric part 2, the rotary cam 6 has three cam projections at equal spaces in circumferential direction, and it is rotatably journaled the roller 8 on each cam projections 6. A cam surface 16 which is rolled by being brought in contact with each roller 8 is formed on the inner circumferential surface of the cam housing 3, and three operating chambers are defined in the housing 3. The output shaft 1 is rotated three times by one time rotation of the rotary cam 6, four strokes of intake, compressing, expansion, and exhaust are carried out in all operating chambers so as to be generate power.