PURPOSE: To improve the control precision by judging the deceleration state of an engine on the basis of the difference between the average value of the detection value at each prescribed cycle of the internal pressure in a suction pipe and the detection value of the internal pressure in the suction pipe at this time and controlling a secondary intake air adjusting valve according to the result of the judgement.

CONSTITUTION: A control circuit 20 receives each detection value of an absolute intake pressure sensor 10, crank angle sensor 11, cooling water temperature sensor 12, O2 sensor 14, car speed sensor 16, etc., and controls the air-fuel ratio through a solenoid valve 9 installed into the secondary intake air feeding pas sage 8 according to the operation state. The control circuit 20 judges the sharp deceleration when shot air is supplied, gentle deceleration when shot air is not supplied, stationary operation, and acceleration, based on the difference between the detection value of the absolute intake pressure sensor 10 and the average value of the detection value of the absolute pressure sensor 10 in the past at each generation of the interruption signal in a prescribed cycle.