PURPOSE: To enable to operate an engine in a stable manner to a certain extent, by controlling the air-fuel ratio by reading out the opening of a throttle valve directly from a map in case that trouble is caused with a differential-pressure sensor.

CONSTITUTION: A differential-pressure sensor detects the pressure difference on the opposite sides of a throttle valve, and characteristics relating to the engine speed N, fuel supply rate QF/N per one rotation of an engine and opening θ of the throttle valve are stored in a map of QF/N-N. In case that it is detected that some trouble is caused with the differential-pressure sensor, the air-fuel ratio is controlled by reading out the opening θ of the throttle valve from the map and adjusting the throttle-valve opening by an actuator. By employing such a method, it is enabled to operate an engine in a stable manner to a certain extent even if trouble is caused with the differential-pressure sensor.