PURPOSE: To obtain a high torque and high power by offsetting the crank shaft of a crank case provided continuously in a cylinder from a piston rod perfectly and setting so that the power stroke of the crank shaft can exceed a specific angle.

CONSTITUTION: A piston rod 4 whose tip is pivoted to the piston 2 inserted in a cylinder 3 is guided slidably by the bushing 7 inside whose front surface is released and the lower end of the piston rod 4 is pivoted to one end of the connecting rod 11 by a wrist pin 6. The other end of the connecting rod 11 is connected communicatingly to the crank shaft 8 arranged on the position offset from the piston rod 4 sufficiently through a journal bearing 9. The offset of the crank shaft 8 is set so as to be in the range from at least one position where the piston rod long shaft is crossed with the periphery of the crank shaft to at least one position where the long shaft is not crossed with the periphery of the crank shaft and thereby, the power stroke of the crank shaft 8 is set beyond 180°.