PURPOSE: To carry out the correct ignition timing control even if regular gasoline is used in an engine which is designed for using high octane number gasoline by delay-correcting the ignition timing when the use of regular gasoline is judged from the delay quantity of the ignition timing based on the detection of knock.

CONSTITUTION: During the operation of a vehicle equipped with an engine using the high octane number gasoline, a controller 11 judges if the delay quantity of the ignition timing advance angle at present which is controlled according to the knock detected by a knock sensor 5 is larger or smaller than the delay threshold value for the judgement for regular gasoline. When the delay quantity exceeds the threshold value, the used gasoline is judged as regular, and when the result of the judgement continues for a prescribed time, if the intake pressure at present is over the threshold value, the delay correction quantity corresponding to the difference of the ignition timing between the high octane number gasoline and regular gasoline is obtained from a table represented from the intake pressure and the revolution speed. Then, an ignitor 8 is controlled by correcting the ignition timing at present by the delay correction quantity.