Bypass air volume control system for throttle body

A bypass air volume control system for a throttle body includes a throttle body provided with: a valve-body guide hole into which a valve body is slidably fitted and into which atmospheric air is introduced; a flat surface formed by opening one end of the valve-body guide hole; and an air control groove which is opened at an inner periphery of the valve-body guide hole in such a manner as to be opened and closed by the valve body and which is opened at the flat surface, a blocking member being fixed to the throttle body and closing an opening end of the air control groove at the flat surface. The blocking member (32) is formed in such a manner that at least a portion thereof exists inside an inner periphery of the valve-body guide hole (10) on the plane including the flat surface (29), the blocking member (32) functioning as a stopper for restricting closing movement of the valve body (27) when the valve body (27) reaches a fully closed position where the opening of the air control groove (30) at the inner periphery of the valve-body guide hole (10) is fully closed. Accordingly, it is possible to achieve reduction in production cost by reducing the processing cost and reducing the number of operation steps.