A bypass air volume control system for a throttle body, including: a throttle body which has an intake passage with a throttle valve disposed therein, to which drive means is fixed, and which is provided with a valve-body guide hole into which a valve body connected to the drive means is slidably fitted, and into which atmospheric air is introduced, a flat surface formed by opening one end of the valve-body guide hole to face a side opposite to the drive means and disposed along a plane perpendicular to an axis of the valve-body guide hole, an air control groove which is directly 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 directly opened at the flat surface such that the air control groove is continuous with the flat surface, and. a downstream air passage having one end continuous with the air control groove and the other end continuous with the intake passage downstream of the throttle valve; and a blocking member (32, 41) which has an annular appearance and is fixed to the throttle body by press-fitting so as to close an opening end of the air control groove at the flat surface, wherein the air control groove and the valve-body guide hole are formed directly in the throttle body; and wherein the blocking member (32, 41), is formed in such a manner that at least a portion thereof exists inside the inner periphery of the valve-body guide hole on the plane including the flat surface, the blocking member (32, 41) functioning as a stopper for restricting closing movement of the valve body when the valve body reaches a fully closed position where the opening of the air control groove at the inner periphery of the valve-body guide hole is fully closed. a throttle body which has an intake passage with a throttle valve disposed therein, to which drive means is fixed, and which is provided with a valve-body guide hole into which a valve body connected to the drive means is slidably fitted, and into which atmospheric air is introduced, a flat surface formed by opening one end of the valve-body guide hole to face a side opposite to the drive means and disposed along a plane perpendicular to an axis of the valve-body guide hole, an air control groove which is directly 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 directly opened at the flat surface such that the air control groove is continuous with the flat surface, and. a downstream air passage having one end continuous with the air control groove and the other end continuous with the intake passage downstream of the throttle valve; and a blocking member (32, 41) which has an annular appearance and is fixed to the throttle body by press-fitting so as to close an opening end of the air control groove at the flat surface, wherein the air control groove and the valve-body guide hole are formed directly in the throttle body; and wherein the blocking member (32, 41), is formed in such a manner that at least a portion thereof exists inside the inner periphery of the valve-body guide hole on the plane including the flat surface, the blocking member (32, 41) functioning as a stopper for restricting closing movement of the valve body when the valve body reaches a fully closed position where the opening of the air control groove at the inner periphery of the valve-body guide hole is fully closed.