PURPOSE: To enhance operability so as to reduce fuel consumption, by adjusting the range of the up and down strokes of a piston in response to the output and engine speed of an internal combustion engine and interposing a connecting rod, in a specific construction, by which the capacity of a combustion chamber is varied, between a pistonarm and crankshaft.

CONSTITUTION: An arm 3 coupled to a vertically movable piston 2 in a cylinder is coupled to a crankshaft by a connecting rod 6. The connecting rod 6 is provided with a cut hole part 7 form in its longitudinal direction, and the movable shaft of a connecting rod 8 is fitted in the cut hole part 7. The connecting rod 8 is rotatably supported by a engine body at its shaft parts 10 positioned on both end thereof, and is set to be moved in the C' direction according to an operating condition. And the range of the up and down strokes of the piston 2 is changed according to the movement of the connecting rod 8 through the connecting rod 6 so that the capacity of a combustion chamber can be varied.