To eliminate a dead point in the vicinity of a point of time of ignition/ explosion and generate large rotational force by having a first part extending obliquely for the direction of reciprocating motion of a piston assembly and second part extending by forming a different angle from that of the first part by a cam groove engaged to an eccentric rotary member.

A cam groove center part 13A constituting both end parts 13B, 13C and a cam groove 13 forms a substantially oblique angle of 45° for the reciprocating moving direction of a piston 10 within a cylinder inner surface 25, and both the end parts 13B and 13C of the cam groove 13 are constituted so as to be substantially orthogonally crossed with each other in the vertical direction. The connecting part of a center part 13A of the cam groove 13 and both the end parts 13B and 13C of the cam groove 13 is formed into a smoothly linked curved shape. Therefore, because a crank pin 38 is engaged to a tip part 13C of the cam groove 13 which is at a substantially right angle to the vertical direction while force pressing the piston 10 downward is concentratedly generated, succeeding ignition/explosion in the vicinity after a top dead point is passed, an excellent transmission of driving force (a conversion into rotational force) can be performed.