A vehicle security system including an intrusion determining unit that transmits a transmission wave in a vehicle, and determines, based on an output of a reflected wave of the transmission wave, whether or not a body intrudes into the vehicle. The security system also includes an adjacent body detecting unit that detects a body adjacent to the vehicle. The intrusion determining unit determines that the body does not intrude into the vehicle, when the output of the reflected wave is within a predetermined range and the adjacent body detecting unit detects the body adjacent to the vehicle.