Abstract: Theft prevention apparatus for vehicles which includes a movement detector provided with a pointer and generating an electric potential when the pointer is moved, a transmitter connected to the movement detector and a receiver provided with a loudspeaker for being adapted to produce an alarm sound. The movement detector and the transmitter are mounted on a vehicle, and the receiver is detachably connected to the transmitter. When the receiver is detached from the connection with the transmitter, the transmitter transmits alarm signal to the receiver in response to the predetermined amount of electric potential generated by movements of the pointer of the detector which is adapted to detect movements of the vehicle. When the receiver is connected to the transmitter, the pointer of the detector is locked against its movements, whereby proper and reliable operation of the detector can be assured.