Motorcycle and theft reporting device installed in the motorcycle

To detect removal of a battery and enhance accuracy of theft detection for a vehicle, a theft detection device uses a voltage monitoring unit for monitoring a voltage of the battery to calculate a drop amount of the voltage in a predetermined time interval (T1). The theft detection device determines whether or not the vehicle is stolen, based on whether or not the drop amount of the voltage has exceeded a predetermined drop amount threshold. The theft detection device includes a radio communication device for transmitting positional information of the vehicle when it is determined that the vehicle is stolen, to thereby report that the vehicle is stolen.