To provide a rocker arm of a design effective to avoid occurrence of a loosening of the adjustment screw to ensure a proper operation of the internal combustion engine including the control of the opening of the valve body on the engine cylinder head, the rocker arm 1 is of a type capable of being rockingly driven by a cam 2 for selectively opening and closing a valve of an internal combustion engine. The rocker arm 1 includes an arm body 4 prepared from a steel plate by means of a press work to have a generally inverted U-shaped section. The arm body 4 has one end formed with an internally threaded hole 12 into which an adjustment screw 7 is threaded. This adjustment screw has one end defining a pivot piece 7b or a valve drive piece 7Ab. Two nuts 13 and 14 are threadingly mounted in overlapping relation to each other on one end portion of the adjustment screw 7 protruding outwardly from the arm body. Instead of the use of the two nuts 13 and 14, a flanged nut may be employed or a single nut 13 in combination with a washer 17 may be employed.