PURPOSE: To use compressed gas source as power effectively by providing at least three reciprocating movement members arranged around a nutation shaft, a cylinder means corresponding to each reciprocating movement members and a means for converting nutation movement to rotary movement of an output shaft.

CONSTITUTION: In a rigid nutation frame on which arms 10, 12 perpendicular to each other are fastened by bolts, the center CF of the arms 10, 12 draws a circle around the rotary axis of an output shaft 24 due to the nutation action of frames 10-12 around a nutation point NP. A nutation shank 28 is mounted on the frames 10-12. The other end of the shank is supported by a bearing 36 on a block 38 mounted on the output shaft 24. The output shaft 24 is supported by a bearing 42 on a stator frame 44. Cylinders C1-C4 are arranged on the frame 44. Pistons P1-P4 are engaged in the cylinders C1-C4. The end of a piston rod 54 is carried by elastomer in a chamber 49 in a block 48 fixed on the arms 10-12.