To perform more reliable judgement of a combustion state by suitably distin guishing the generation of carbon fouling of an ignition plug and preignition.

An engine 11 is provided with an ignition coil, and ignition is performed by an ignition plug 51 by allowing primary current to flow to a primary coil. Leakage current is not easily generated between electrodes of the ignition plug 51 in the early stages of primary current energization when carbon fouling (sticking of carbon to the electrodes of the ignition plug 51) is generated, and the finishing timing of the generation of leakage current is delayed to the termination of primary current energization as the smoke is progressed. In the generation of preignition, ionic current is generated between the electrodes of the ignition plug 51 in the termination of primary current energization, and the starting timing of the generation of ionic current is advanced to the early stages of the primary current energization as preignition is progressed. Currents between the electrodes of the ignition plug 51, in the early stage and the termination of energization of primary current are detected, the number of current detections is counted, and the combustion state of a combustion chamber 16 is accurately judged on the basis of the counted values.