PURPOSE: To enable the high control of high speed responsiveness, by providing a first throttle valve provided with a cylindrical passage for allowing flow of intake air of an engine, a second throttle valve for opening/closing this cylindrical passage, and an air leakage preventing pipe for preventing air from leaking from leaking from the circumference of the first throttle valve.

CONSTITUTION: A first throttle valve 12 provided with a cylindrical passage 13 for allowing flow of an intake air, is provided rotatably in an intake air passage 11 and a gear 14 is provided on the projection end of the shaft part 11a of said valve 12. A valve stem 15 is mounted on the rotational center shaft line of this throttle valve 12, a second throttle valve 17 for opening or closing the cylindrical passage 13 is provided on this valve stem 15, while a gear 16 is provided on the projection end part of the valve stem 15. Prime mover gears 18, 19 rotated by driving motors 20a, 21a are meshed with the respective gears 14, 16. On the other hand, a cylindrical air leakage preventing pipe 31 is fitted inside an intake pipe part 29 in order to close a gap 30 between the circumferential surface of the first throttle valve 12 and the curve shoulder part inner surface from a neck-type part to a barrel-type part in the intake pipe part 29.