An ignition timing setting apparatus includes a pulse generator for generating crank pulses corresponding to crank angles, a crank angular speed variation calculating section for calculating a crank angular speed variation based on an interval of the crank pulses, an engine load estimating section for estimating an indicated mean effective pressure from the crank angular speed variation, and an ignition timing determining section having an ignition timing control map for determining an ignition timing advance quantity in accordance with the estimated indicated mean effective pressure and an engine temperature or an engine speed. Such ignition timing setting apparatus sets appropriate ignition timings for avoiding a knocking occurrence load range based on an accurate estimated engine load.