PURPOSE: To gradually perform the decrease and the increase of intake quantity so as to stabilize engine performance by arranging a control inner wall face which maintains the gap less than the specified opening of the throttle valve in continuation to a closed inner wall face which sets a gap with respect to the outer peripherary of a throttle valve at the time of idling.

CONSTITUTION: Respective intake pipes 2,3 are connected to an entrance and an exit sides of body 1, and a throttle valve 5 is supported, through a throttle valve shaft 4, at the center of the body 1. By this constitution, when the throttle valve 5 is at the idle position X-X, a closed inner wall face 6 with which both outer peripheraries 5a, 5b are, respectively, brought into contact is formed at ventilation passage walls of the body 1. At the passage walls of the respective intake pipes 2,3, the respective control inner wall faces 8,10 which maintain a gap between both the outer peripheraries 5a, 5b, respectively, are formed continuously to the closed inner wall face 6 at the symmetrical position around the throttle shaft 4, with the level of opening less than the specified opening of the throttle valve 5.