PURPOSE: In an internal-combustion engine where a negative pressure sensor is employed to detect the idling, to always achieve the suitable air-fuel ratio control, even when the erroneous idling is detected, by arranging a delay element at the downstream of a regulator provided in an intake manifold.
CONSTITUTION: A proportional control valve 10 provided in an air guide tube 9 conducting to an intake manifold 2 while detouring a carburetor 7 is feedback controlled by an oxygen concentration sensor 13 provided in an exhaust manifold 4 through a computor 14. Said control is stopped at the idling and the starting of an engine by a negative pressure sensor 16 provided in the intake manifold 2 and a regulator 17. In such controller a delay element 18 is arranged at the downstream of the regulator 17 to properly delay the time of a signal at the cranking decided by the regulator 17. Consequently even when said sensor 16 can not detect the idling under low atmospheric pressure at highland or the like, concentrated mixture gas can be fed for predetermined time.