To enrich an antitheft mechanism without using radio waves while maintaining a mechanism of electronically collating ID codes.

When a key plate 1 is inserted into a key cylinder 4, power source is supplied to an IC chip 3 to light on an LED 2. The LED 2 is modulated to an ID pulse signal by a light-up pulse, depending on the inherent ID code stored in an IC chip 3. The modulated LED light is received by a PD 5, then amplified by an amplifier 6, and thereafter transmitted to an engine controller computer 1. Then, the engine controller computer 7 wave-shapes the received pulse signal, and thereafter a decoder decodes the signal so as to coordinate the ID code. Further, the ID code from the received pulse signal is collated with the ID code stored in a ROM of the engine controller computer 7 to determine as to matching. If it is determined that the two ID codes match each other, a signal is transmitted to an action section 8 to start an engine.