PURPOSE: To smoothly shift the slow operation to rapid operation of an engine having a plurality of intake passages for one cylinder, by providing a sliding throttle valve, which moves across the intake passages to sequentially open and close them.

CONSTITUTION: Two intake valves 20 and two exhaust valves 22 are provided for one cylinder. A sliding throttle valve 60 is provided with a hollow part 64 so that the valve can move across the intake passages 56, 58 of each cylinder. A valve plate 62 is slidably fitted. The intake passages 56, 58 are sequentially opened and closed by causing the valve plate 62 to slide. Since the shifting of slow operation to rapid operation is effected only by the sliding of the single valve plate 62, the shifting is smooth and steady.