To suppress eccentric wear of an intake and exhaust valve of an overhead valve engine.

An intake and exhaust valve 12 is mounted on a cylinder head 3, the operation of a cam provided in a cylinder block is converted into oscillation of a rocker arm 6 in the vertical direction through a push rod 9, and an output end part 10 of the rocker arm 6 is brought into contact with an upper end part 17 of a valve stem 13 of the intake and exhaust valve 12 to open and close the air suction and exhaust valve 12 by lifting, lowering, and driving the intake and exhaust valve 12. The output end part like a tapered face of the rocker arm 6 comes into point-contact with the upper end part 17 like a projection spherical face of the intake and exhaust valve 12. The contact point is deviated from the center of an axis of intake and exhaust valve 12, and the intake and exhaust valve 12 rotates around the center of an axis thereof due to the oscillation of the rocker arm 6.