PURPOSE: To secure a prescribed reception sensitivity irrespective of the direction of a detection area by providing a gain control means which varies the gain of a reception amplifier of a heatwave-type human body detecting sensor in accordance with the direction of the detection area.

CONSTITUTION: A heatwave-ultrasonic composite type burglar sensor has a construction wherein an ultrasonic-type sensor unit 1 and a heatwave-type sensor unit 2 are combined with each other by a signal processing circuit 3. An invader is detected by the signal processing circuit 3 on the basis of an output of each of the sensor units 1 and 2 and an alarm output means 7 is driven by an output of this signal processing circuit 3. The aforesaid ultrasonic-type sensor unit 1 is of a Doppler type, and a transmitter 11 is driven by an output of an oscillator 10 to transmit an ultrasonic wave, while reflection by an object is received by a receiver 12. An output of the receiver 12 is amplified by a received-wave amplifier 13, a reception signal thus obtained is compared with a transmission signal, the output of the oscillator 10, a frequency shift is detected by a frequency shift detector 14 on the basis of the comparison, and the presence of the invader is detected from this frequency shift.