An ignition system is provided, which can restrict decreasing of constant-voltage duration of a spark plug and effectively prevents the occurrence of an accidental fire in an engine. A typical ignition system includes a secondary coil having one end connected to a positive side of a battery via a low-voltage side path and the other end connected to a center electrode via a connecting path which connects the secondary coil and the spark plug. A constant-voltage path having a grounded end is connected to the connecting path. A block diode is arranged between the secondary coil and a point where the constant-voltage path is connected with the connecting path. A Zener diode is disposed within the constant-voltage path. Each anode of the block diode and the Zener diode is mutually connected.