To improve durability by reducing load acting on a camshaft, a timing gear and a timing belt by keeping constant rotation speed of the camshaft even if a crankshaft is rotated at non-uniform speed.

A pair of ellipse gears 25, 26 meshing each other are arranged between the crankshaft 6 and an output shaft 24. When periodic non-uniform speed rotation of the crankshaft 6 is converted to constant speed rotation and is transmitted to the output shaft 24 by rotation of both of the ellipse gears 25, 26, a drive side timing gear 27 for camshaft drive is pivotally supported by the output shaft 24, a driven side timing gear 28 is pivotally supported by the camshaft 22, the both gears 27, 28 are connected by the timing belt 29, and the camshaft 22 is driven by constant speed rotation of the output shaft 24.