It is an object of the invention to provide a vehicle alarm signal generation device which can realize the reduction of the consumption of power without unstabilizing the intruder detect operation. The vehicle alarm signal generation device includes a supersonic sensor which comprises a supersonic transmitter and a supersonic receiver. The supersonic transmitter radiates supersonic waves in the interior of a vehicle and the radiated supersonic waves are received by the supersonic receiver. If any person intrudes on the vehicle interior, then there is produced a difference between the frequencies of the radiated and received supersonic waves due to the Doppler effect generated responsive to the movements of the intruder, so that a beat frequency signal corresponding to the frequency difference is included in a signal received by the supersonic receiver. The beat frequency signal is integrated by an integrating circuit and is then converted into a voltage signal. A control circuit is arranged such that it generates an alarm signal when the voltage signal arrives at or goes beyond a reference level. The control circuit drives the supersonic sensor intermittently, invalidates the voltage signal for a given time after it starts the driving of the sensor and, after lapse of the invalidated time of the voltage signal, switches the supersonic sensor into its successive driving condition when the voltage signal reaches or exceeds an auxiliary reference level lower than the reference level.