An ignition system has an ignition coil unit, an ignition plug, and a voltage limiter. The ignition coil unit has a primary coil and a secondary coil which are magnetically coupled with each other. The ignition plug has a center electrode and a ground electrode. The ignition plug generates discharge spark between the center and ground electrodes to which voltage is applied on the basis of the magnetic energy accumulated in the ignition coil unit. The voltage limiter limits an absolute value of the voltage applied to the center and ground electrodes of the ignition plug to a voltage limit value. The voltage limiter increases the voltage limit value as cumulative usage time of the ignition plug increases.