The present invention provides a burglar alarm system capable of accurately discriminating an alarm from a disturbance noise approximate to the alarm such as background music, ambient noises, etc. In a burglar alarm system comprising an alarm unit attached to an object to be protected from theft to emit an alarm of a particular frequency and an alarm sensor device for issuing an alarm signal upon sensing an alarm from the alarm unit, the alarm sensor device counts a predetermined number of waveform pulses of a received sound (S2, S5) more than once (S1.about.S3, S4.about.S6), determines, if it transpires that a differential between counted values falls within a predetermined time range, that the received sound is an alarm and issues an alarm signal (S7).