In a bypass intake control system, a bypass valve (5) is formed from a valve housing (6) having a valve hole (10), an inlet port (11) connecting to the upstream side of a bypass (4) and an outlet port (12) connecting to the downstream side of the bypass (4), and a valve piston (13) fitted in the valve hole (10) in a slidable and non-rotatable manner, the valve piston (13) being linked to an output shaft (23) projecting out of an end surface of a rotor (22) of a stepping motor (14) via a screw mechanism (29) on the side opposite to the bypass (4) relative to sliding surfaces of the valve housing (6) and the valve piston (13). It is thereby possible to prevent the screw mechanism from being exposed to backfire gas. The engine negative boost pressure does not impose a high load on the stepping motor thus making it possible to reduce the capacity of the motor.
