PURPOSE: To reduce the manhour of work by providing flat surfaces which extend in a sliding direction and are parallel to each other, on the both ends in the width direction perpendicular to the direction of intake air, and castingly molding the captioned valve in such a way that parting lines are formed on said flat surfaces.

CONSTITUTION: A sliding throttle valve having a long elliptical cross section perpendicular to the direction 8 of intake air of an intake air passage, slides in a guide cylinder 7 to the bottom end of which a needle 38 is fixed, and which has an inner surface of nearly the same configuration as the valve. The throttle valve is cast in metal molds 51 and 52 when manufactured by casting and, by setting its form in such a way that flat surfaces 35, 36 are provided on both ends in its width direction, parting lines at the time of removing the metal molds, are formed on said flat surfaces 35, 36, entering within an end face circular arc 33. Accordingly, the throttle valve can slide in the end face circular arc 33 of the guide cylinder 7 without being affected by the parting lines in the least.