A theft prevention device makes an ECU operable by transmitting a second ID code to the ECU from a keyset when an input code agrees with a first ID code. A device is provided for inputting a second reference code which is collated with a second ID code provided to a receiver unit of an ECU or a keyset. A device is provided which, when the inputted code and a pre-registered second reference code agree with each other, writes the registered second reference code into the receiver unit as the second ID code. The code which is inputted as the second reference code is inputted based on an opening/closing manipulation of a throttle provided to a vehicle and a plurality of times of standing/storing manipulation of a side stand with and one digit of the code being represented by the number of throttle manipulations during two side stand standing/storing manipulations.