PURPOSE: To decrease exhaust gas effectively in a broad range, by learning and correcting an air-fuel ratio correcting coefficient at every running speed based on the difference between the actual air-fuel ratio in return control obtained by an O2 sensor and the predetermined basic air fuel ratio.

CONSTITUTION: A control circuit receives the detected values from an air intake quantity sensor, an engine speed sensor, a throttle opening degree sensor, the O2 sensor provided in an exhaust gas system, a vehicle speed sensor, and the like, computes the basic injection time of a fuel injection value from the air intake quantity and the engine speed, and performs the return control so as to obtain a specified air-fuel ratio in response to the output of the O2 sensor. The learning and correction of the air-fuel ratio correction coefficient is performed for the vehicle speeds of 0, 20, 40, 60, 80, and 100km/hr, in response to the deviation between the actual air-fuel ratio in the return control and the specified air-fuel ratio. The learned correction values KG1WKG6 are stored in a non-volatile memory and the suitable air-fuel ratio control is performed from the beginning at the next running.