PURPOSE: To improve a rate of fuel consumption as well as to reduce a harmful exhaust gas component, by controlling an air bypass flow control valve with a feedback value of an air-fuel ratio controlling device and setting an air-fuel ratio down to the vucinity of a desired air-fuel ratio.

CONSTITUTION: A suction manifold 9 is provided with a main suction passage 10 and an auxiliary suction passage 11 consisting of a smaller sectional area than the passage 10. An air bypass flow control valve 19 is installed in the auxiliary suction passage 11. An electronic-control unit 22 compares a desired air-fuel ratio with the exhaust air-fuel ratio detected by a lean sensor 24, and outputs an air-fuel ratio feedback value made by the comparative difference to the air bypass flow control valve 19, thus the exhaust air-fuel ratio comes to the vicinity of the desired air-fuel ratio. Thus, a rate of fuel consumption is improvable and, what is more, a harmful exhaust gas component is reducible.