In an ignition system wherein an ignition timing signal generated in synchronism with the engine rotation is detected by a detecting transistor and a base current of a power transistor adapted to control a primary current of an ignition coil is controlled by the detected ignition timing signal thereby to generate a high voltage across a secondary side of the ignition coil; the primary current of the ignition coil is controlled by the power transistor not to exceed a predetermined value, a time interval during which the primary current of the ignition coil is restricted to the predetermined value is detected by a change in collector voltage of the power transistor, i.e. an output thereof, and a conduction starting point for the primary current of ignition coil is controlled by the output of the power transistor.