PURPOSE: To improve fuel consumption amount and drivability by controlling air- fuel ratio according to fluctuation of pressure in combustion chamber.

CONSTITUTION: A combustion pressure detecting means 18 takes out pressure based on output of a pressure sensor 9, as a sample value, according to crank position indicated by output of a crank position detecting means 19, while said output being known, by signal of a crank angle sensor, 10. Mean effective pressure as typical value showing pressure behavior and its fluctuation can be calculated in a mean effective pressure arithmetical means 20. An air-fuel ratio control means 21 consists of an arithmetical means 22 for known fuel basic injection amount indicated by signals from an air flow meter 4 and other sensors and an arithmetical means 23 for corrected fuel injection amount operating based on output of the mean effective pressure arithmetical means 20, and a drive signal generating means 24 to fuel injection valve 6 is connected to the above means 22 and 23. Then fuel consumption amount and drivability can be improved.