To accurately eliminate reduction in an engine speed caused when reducing fuel supply quantity, without reducing fuel consumption, in an internal combustion engine for controlling the quantity of fuel supplied for converging the air-fuel ratio in the target air-fuel ratio.

The internal combustion engine controls the quantity of fuel supplied for converging the detected air-fuel ratio in the target air-fuel ratio, by detecting the air-fuel ratio; and is controlled on the ignition timing for reducing a variation in a signal, by indexing a frequency component substantially coincident with a period of a change in the quantity of fuel supplied from a change with the lapse of time of the signal for indicating the detected engine speed, by detecting a change with the lapse of time of the engine speed of an engine of an internal combustion engine.