An ignition system is provided, in which a Zener diode is connected in parallel with a spark plug, to suppress torque variation from becoming large in an engine when deterioration of the spark plug is advanced. Specifically, the ignition system includes a secondary coil having an end connected to a center electrode of the spark plug via a connecting path. The connecting path is connected to a constant-voltage path having a grounded end and including the Zener diode. The time from when an ignition signal is switched off until when ignition timing occurs is measured for a plurality of times. The difference between a maximum value and a minimum value among the plurality of measurements is defined to be a variation range. A breakdown voltage of the Zener diode is adjusted based on requirements such as for rendering the variation range to be a predetermined time or smaller.