In a speed transition period from when an internal combustion engine is started to when an engine speed settles down to a certain speed, a compression top dead center of a cylinder first operating is set as a reference crank angle and compression top dead centers of the cylinders arriving successively after said reference crank angle in the speed transition period are set as judgment use crank angles. Reference crank angle advancing times are detected and stored in advance, wherein the reference crank angle advancing times are crank angle advancing times when a reference fuel is used, and the crank angle advancing times are times required for the crank angle to advance from the reference crank angle to the judgment use crank angles. The actual crank angle advancing times are detected. When an actual crank angle advancing time is shorter than the reference crank angle advancing time, the ignition timing is retarded by an amount corresponding to a difference of the actual crank angle advancing time from the reference crank angle advancing time. When the actual crank angle advancing time is longer than the reference crank angle advancing time, the ignition timing is advanced by an amount corresponding to the difference.