An ignition timing control apparatus for a spark ignition internal combustion engine which injects fuel directly into a combustion chamber, comprising: an injection timing detection unit for detecting a fuel injection timing from a control signal; a rotation speed detection unit for detecting an engine rotation speed; a load detection unit for detecting an engine load; a storing unit for storing first control data of an amount of change in an optimal ignition timing with respect to an engine rotation speed and a fuel injection timing and second control data of correction coefficient of the optimal ignition timing with respect to an engine load; and a control unit for calculating an optimal ignition timing based on the fuel injection timing detected by the injection timing detection unit, the engine rotation speed detected by the rotation speed detection unit and the engine load detected by the load detection unit and controlling an ignition timing of an ignition plug to the optimal ignition timing, wherein the control unit calculates: a first amount of change in the optimal ignition timing by applying the detected fuel injection timing and the detected engine rotation speed at a predetermined load to the first control data; a second amount of change in the optimal ignition timing by applying a previously determined, specific fuel injection timing and the detected engine rotation speed at a predetermined load to the first control data; a deviation between the first amount of change and the second amount of change; a correction coefficient with respect to the detected engine load by referring to the second control data of the correction coefficient; an amount of change in the optimal ignition timing after correction based on the deviation and the correction coefficient. an injection timing detection unit for detecting a fuel injection timing from a control signal; a rotation speed detection unit for detecting an engine rotation speed; a load detection unit for detecting an engine load; a storing unit for storing first control data of an amount of change in an optimal ignition timing with respect to an engine rotation speed and a fuel injection timing and second control data of correction coefficient of the optimal ignition timing with respect to an engine load; and a control unit for calculating an optimal ignition timing based on the fuel injection timing detected by the injection timing detection unit, the engine rotation speed detected by the rotation speed detection unit and the engine load detected by the load detection unit and controlling an ignition timing of an ignition plug to the optimal ignition timing, wherein the control unit calculates: a first amount of change in the optimal ignition timing by applying the detected fuel injection timing and the detected engine rotation speed at a predetermined load to the first control data; a second amount of change in the optimal ignition timing by applying a previously determined, specific fuel injection timing and the detected engine rotation speed at a predetermined load to the first control data; a deviation between the first amount of change and the second amount of change; a correction coefficient with respect to the detected engine load by referring to the second control data of the correction coefficient; an amount of change in the optimal ignition timing after correction based on the deviation and the correction coefficient. a first amount of change in the optimal ignition timing by applying the detected fuel injection timing and the detected engine rotation speed at a predetermined load to the first control data; a second amount of change in the optimal ignition timing by applying a previously determined, specific fuel injection timing and the detected engine rotation speed at a predetermined load to the first control data; a deviation between the first amount of change and the second amount of change; a correction coefficient with respect to the detected engine load by referring to the second control data of the correction coefficient; an amount of change in the optimal ignition timing after correction based on the deviation and the correction coefficient.