PURPOSE: To increase accuracy in detecting the flow rate of intake air, by providing an air valve in an intake pipe at the portion thereof located between a supercharger and an intake-air detecting means, detecting the opening of the air valve according to the operational conditions of an engine, and moderating pulsation of intake air transmitted from the side of the supercharger and the side of the intake-air detecting means.

CONSTITUTION: At the time of low-speed operation of an engine 7, the flow rate of intake air through an intake pipe 5 is low and the flow rate of exhaust gas through an exhaust pipe 12 is also low, so that the speed of rotation of a supercharger 3 is low. In particular, in case that a throttle valve 9 is fully opened, pulsation of intake-air flow is strong, so that bad influence is exerted to an intake-air detecting means 2. In order to overcome the above problem, an electronic control unit 13 is operated by an output signal of said detecting means 2 to close an air valve 4 by driving a solenoid valve 6 and to thereby minimize transmission of pulsation of intake air to said detecting means 2. Further, at the time of deceleration when a throttle valve is closed rapidly, intake air pressurized by the supercharger 3 is reflected off the throttle valve 9 and flows back, so that a rapid pressure rise is transmitted to said detecting means 2. In order to prevent transmission of the rapid pressure rise to the detecting means 2, the air valve 4 is closed by the output signal of a valve opening detector 10.