PURPOSE: To compose wave forms with a simple construction by providing two combined circuits which transmit an engine rotation synchronous signal to the trigger input terminal and output terminal of a CDI, on an ignition signal wave form composing circuit, in a car engine, etc.

CONSTITUTION: A signal synchronized with the rotation of an engine is given to the terminal 16 from a transistor type ignition signal generator, and enters the terminal 38a of the transformer 38 of a CDI through a diode 22, turning on a thyristor 36. The charge of the capacitor 34 of the CDI is discharged through the thyristor 36, and a current flows into the primary side 18P of an ignition coil 18 from a battery 20 to generate a high voltage in its secondary side 18S. On the other hand, a signal from the terminal 16 flows into the primary side 18P through a diode 24, and an output wave form from the CDI is completely overlapped with a wave form inputted into the terminal 16.