Abstract: A mechanism for rotating a crankshaft at ununiform speeds through a main gear and a slider mechanism and a mechanism for rotating a crankshaft at a uniform speed through a main gear fixed to the crankshaft are constructed such that the distances between the centers of rotation of respective main gears and the centers of respective drive shafts are the same. A speed reducing ratio between a drive gear mounted on the drive shaft and the main gear is set to be the same, and the positions of the crankshaft and the drive shaft are set to be the same for both the mechanisms. When rotating the crankshaft at ununiform speeds, a lever provided for the slider mechanism is disposed to orient 180 degree out of phase with respect to the eccentric direction of a crankpin. This obviates a rotation balancing weight which has been provided on the crankshaft in the prior art.