To provide good speed responsiveness of an engine by increasing an amount of air in accordance with opening action of a sliding throttle valve, in a region to a fine degree opening from a fully closed condition of the sliding throttle valve.

A sliding throttle valve 20 holds an intake path 2 fully closed, and an intake path in the upstream from the sliding throttle valve 210 and an intake path 2C on the engine side are connected by a bypass passage. A notched hole 20C is drilled in an engine side opposed side surface 20A of the sliding throttle valve 20. This notched hole 20C is positioned on the side of a perpendicular X-X of passing through a center A of a cross section of the intake path 2, to be drilled upward from a lower end 20B along the perpendicular X-X. In a fully closed condition of the intake path 2 by the sliding throttle valve 20, a part 20C' of the notched hole 20C is opened toward the inside of the intake path 2.