To control air-fuel ratio and ignition timing to a combustion stable limit without being influenced by dispersing of a combustion pressure detection value between cylinders.

A cylinder internal pressure is integrated by a prescribed integration interval in each cylinder, to obtain a cylinder internal pressure integration value Pi to be classified by cylinder (S1), and a change factor ΔPi of the integration value Pi is calculated to be classified by each cylinder (S2). In a range of not exceeding a prescribed value in the change rate ΔPi by classifying each cylinder (S3), ignition timing is delayed or air-fuel ratio is changed to be lean (S4).