PURPOSE: To prevent blow back of exhaust gas in the exhaust passage side to the air intake passage side even if intense negative pressure is caused in the air intake passage side by making overlap, due to simultaneous opening of an air intake valve and an exhaust valve, smaller at low speed than that at high speed.

CONSTITUTION: When an engine is in high speed operation, since a sprocket shaft 20a is in high speed rotation, a steel ball 24 is moved so as to go apart from an axis by means of centrifugal force, and a movable plate 23 is moved rightward, further a fork-like lever 29 supported by a base plate 19 is pushed rightward. Said lever 29 aways using, as a fulcrum, a point P of its engagement with the end part of a groove 25a of support plate 25 fixed to a cylinder head, and the base plate 19 is moved upward. During low speed operation such as idling and the like, since centrifugal force of the steel ball 24 is small, the base plate 19 is moved downward and the axial center of the sprocket shaft 20a shifted from the axial center of a cam shaft 13 by an interval (e), therefore, the cam shaft 13 makes unequal speed rotation against rotation of the crankshaft, overlap of the intake and exhaust valves being reduced.