A theft prevention apparatus of a leisure vehicle equipped with a rider's seat which opens outside is disclosed. The theft prevention apparatus typically includes a portable transmitter configured to transmit a user identification code by radio at predetermined intervals during travel of the vehicle, a vehicle receiver mounted in the vehicle and configured to receive the user identification code which is transmitted by radio from the portable transmitter, a controller coupled to the vehicle receiver through a signal line or by radio, and an alarm device mounted in the vehicle and configured to indicate an alarm to inform the rider that the portable transmitter has been lost, wherein the controller is configured to determine whether or not the vehicle receiver has received the user identification code transmitted from the transmitter within a predetermined time during travel of the vehicle, and to execute control to cause the alarm device to indicate the alarm when the controller determines that the receiver does not receive the user identification code within the predetermined time.