To improve durability of a valve cap which is brought into push- contact with a rocker arm.

A valve push part 7 of the rocker arm 1 has a valve push contact surface 7a on a bottom surface, the valve push contact surface 7a is formed in a projection curve surface gently downwardly along the longitudinal direction of the rocker arm 1, and is provided with a groove 9. The groove 9 is formed into a rectangular shape, a center line L1 in the width direction of the rocker arm 1 of the groove 9 passes the groove 9, and a center line L2 in the width direction of the rocker arm 1 of the groove 1 is positioned shifted from the center line L1 in the width direction of the rocker arm 1 of the valve push contact surface 7a. A cylindrical valve cap 11 is mounted on an end part of the rocker arm 1 side of a valve stem 10, positioned directly below the valve push part 7 of the rocker arm 1 freely rotatably about the valve stem 10, and a center of the valve cap 11 is arranged on the center line L1 in the width direction of the rocker arm 1 of the valve push contact surface 7a of the rocker arm 1.