PURPOSE: To reduce the size of an engine and to enable to take out turning force with no use of a conventional crank mechanism, by disposing a cylindrical member fixed to an output shaft in a piston disposed in a cylinder in a freely slidable manner only in the axial direction by the intermediary of rolling balls and cam-like guide grooves.

CONSTITUTION: A cylindrical member 10 is fixed, by the intermediary of a fixing member 11, to one end portion of an output shaft 1 located on the side of a cylinder 3, and the output shaft 1 is journalled in a freely rotatable manner at the bottom 12 of the cylinder 3. A plurality of guide grooves 9 (91W96) having an arcuate cross section are formed in the outer surface of the cylindrical member 10 substantially at the circumferentially balanced positions, and rolling balls 8, 8' and so on are put in these grooves 9, i.e., one ball for each groove. These balls 8 are supported in a freely rotatable manner at proper positions on the inner cylindrical portion of a piston 5. Further, an axial groove 7' is formed in the outer cylindrical portion of the piston 5 and a spherical projection 7 fitted in the axial groove 7' is formed at a proper position on the inner surface of the cylinder 3 for preventing rotation of the piston 5. The cylinder 3 is formed with a scavenging passage 19 communicating a compression chamber 13 and a combustion chamber 20 with each other.