A control device for an internal combustion engine that can be operated with a different air-fuel ratio at each of a plurality of cylinder groups, and is configured to control quantities of air that is taken into respective cylinders of the plurality of cylinder groups with one throttle, comprising: request torque acquiring means for acquiring request torque to the internal combustion engine; request air-fuel ratio acquiring means for acquiring a request air-fuel ratio to each of the plurality of cylinder groups; reference air-fuel ratio setting means for setting a reference air-fuel ratio within a range from the leanest air-fuel ratio to the richest air-fuel ratio out of the request air-fuel ratios to the respective cylinder groups; target air quantity calculating means for calculating a target air quantity for realizing the request torque under the reference air-fuel ratio, based on data that defines a relation between torque generated by the internal combustion engine and an air quantity in relation to an air-fuel ratio; throttle opening control means for controlling an opening of the throttle in accordance with the target air quantity; fuel injection amount control means (12R, 12L) for controlling fuel injection amounts of the respective cylinders in accordance with the request air-fuel ratios to the respective cylinder groups; estimated torque calculating means for calculating an estimated torque, which is a torque estimated to be achieved under an optimum ignition timing, based on an estimated air quantity calculated from a current opening of the throttle and the reference air-fuel ratio; and ignition timing controlling efficiency calculating means for calculating an ignition timing controlling efficiency, which is a ratio of the request torque to the estimated torque; characterized by further comprising combustion limit guard means for specifying a combustion limit efficiency corresponding to the reference air-fuel ratio, based on a data which defines for every air-fuel ratio a minimum efficiency with which suitable combustion is secured, receiving the ignition timing controlling efficiency from the ignition timing controlling efficiency calculating means, outputting the ignition timing controlling efficiency as it is when the ignition timing controlling efficiency is larger than the combustion limit efficiency, and outputting the combustion limit efficiency as substitute for the ignition timing controlling efficiency when the ignition timing controlling efficiency is equal to or smaller than the combustion limit efficiency; and ignition timing control means for calculating a retard amount with respect to the optimal ignition timing from the ignition timing controlling efficiency or the combustion limit efficiency outputted from the combustion limit guard means, and controlling ignition timing of the respective cylinders in accordance with the retard amount. request torque acquiring means for acquiring request torque to the internal combustion engine; request air-fuel ratio acquiring means for acquiring a request air-fuel ratio to each of the plurality of cylinder groups; reference air-fuel ratio setting means for setting a reference air-fuel ratio within a range from the leanest air-fuel ratio to the richest air-fuel ratio out of the request air-fuel ratios to the respective cylinder groups; target air quantity calculating means for calculating a target air quantity for realizing the request torque under the reference air-fuel ratio, based on data that defines a relation between torque generated by the internal combustion engine and an air quantity in relation to an air-fuel ratio; throttle opening control means for controlling an opening of the throttle in accordance with the target air quantity; fuel injection amount control means (12R, 12L) for controlling fuel injection amounts of the respective cylinders in accordance with the request air-fuel ratios to the respective cylinder groups; estimated torque calculating means for calculating an estimated torque, which is a torque estimated to be achieved under an optimum ignition timing, based on an estimated air quantity calculated from a current opening of the throttle and the reference air-fuel ratio; and ignition timing controlling efficiency calculating means for calculating an ignition timing controlling efficiency, which is a ratio of the request torque to the estimated torque; characterized by further comprising combustion limit guard means for specifying a combustion limit efficiency corresponding to the reference air-fuel ratio, based on a data which defines for every air-fuel ratio a minimum efficiency with which suitable combustion is secured, receiving the ignition timing controlling efficiency from the ignition timing controlling efficiency calculating means, outputting the ignition timing controlling efficiency as it is when the ignition timing controlling efficiency is larger than the combustion limit efficiency, and outputting the combustion limit efficiency as substitute for the ignition timing controlling efficiency when the ignition timing controlling efficiency is equal to or smaller than the combustion limit efficiency; and ignition timing control means for calculating a retard amount with respect to the optimal ignition timing from the ignition timing controlling efficiency or the combustion limit efficiency outputted from the combustion limit guard means, and controlling ignition timing of the respective cylinders in accordance with the retard amount.