An ON-ignition signal is outputted from an ignition control portion in a period from a posterior time point of the power stroke to an valve opening timing of an exhaust valve so that a capacitor is charged. Then, when it is determined that the maximum value of an ion-output value detected during a negative valve overlap period becomes greater than or equal to a threshold, it is determined that the applied voltage between an center electrode and a ground electrode of a spark plug is dropped. The ON-ignition signal is outputted again to charge the capacitor before the intake valve is opened.