PURPOSE: To always perform a correct study in the method of correctively studying a correction coefficient of air-fuel ratio when the air fuel ratio is set in acccordance with a deviation between the detected air-fuel ratio and the target air fuel ratio, by performing the study in every prescribed region within a study range of intake pipe pressure or intake air quantity.

CONSTITUTION: The basic injection timing calculated from an intake pipe pressure sensor 23 and crank angle sensor 44 is corrected in accordance with a deviation between the air-fuel ratio by an O2 sensor 34 and the target air-fuel ratio in a digital control circuit 54. While an air-fuel ratio correction coefficient, used when air-fuel ratio is set in accordance with said deviation, is correctively studied. Here said correction coefficient is studied in every prescribed region within a study range of intake pipe pressure or intake air quantity. Then at air- fuel ratio control, an air-fuel ratio correction coefficient in the upper limit region within the study range is used for a region exceeding the upper limit of said study range and/or an air-fuel ratio correction coefficient in the lower limit region within the study range is used for a region below the lower limit of the study range.