Abstract: A mechanism for computing the basic ignition period of a spark plug from engine running conditions, a mechanism for judging whether or not the running conditions are within a delay correction region, and a mechanism for computing a basic delay amount if the running conditions are within the delay correction region, are provided. The accelerator opening or throttle opening are detected, and after the delay amount has been corrected according to the detected opening, the basic ignition period is corrected by the basic delay amount. The spark plug thereby ignites the fuel with the corrected ignition period, and the engine catalyst is activated early without impairing engine running performance after engine start-up.