To prevent the occurrence of a bicycle theft accident by providing a vibration sensor to detect a micro-vibration, or a photo sensor to make judgement about the light and darkness of daytime and nighttime, further providing a switch for setting an ID code, and operating a flashlight and an intermittently warning buzzer via judgement about the occurrence of a theft, if any.

The knob of an ID code setting switch is provided on the rear part of a body B having a flashlight indication cover A at the front, and the body B is internally provided with a battery space F, a warning buzzer sound pressure space G. a mounting space H for a printed circuit board with a built-in electronic circuit. Also, a vibration sensor to detect a vibration, a photo sensor to identify light and darkness, a light emitting diode for travel indication, and the like are built into the printed circuit board. Then, a vibration generated at the time of moving a bicycle during storage is detected and a theft alarm is generated. The sound of a buzzer is stopped by setting the ID code with the knob. Also, the buzzer is made to sound, and the flashlight is additionally made to flash, regarding a theft accident at night.