A variable valve operating mechanism of a four-stroke internal combustion engine includes a slide pin energized by a pin spring in a direction while oil pressure acting on the slide pin in the opposite direction through oil pressure supply passages. The slide pin moves by controlling the oil pressure causing a stem contact surface and a stem through hole to selectively face a valve stem. The mechanism also includes an oil discharge passage including a discharge port which is opened to allow oil acting the slide pin to be discharged when the valve lifter is pressed by a valve operating cam to move for opening the valve. The mechanism is capable of quickly moving the slide pin upon the release of oil pressure when the valve comes into a quiescent state. Therefore, the response of transitioning the variable valve from an operating state to the quiescent state is improved.