The present invention provides a burglar alarm system capable of accurately discriminating external noises approximate to an alarm such as background music, ambient noises, etc. and reverberations of an alarm attributable to an architectural design of the interior of a store, etc. from a legitimate alarm. A burglar alarm system of the present invention comprises an alarm unit attached to an object to be protected from theft to emit an alarm of a certain frequency in response to illegal conduct and an alarm sensor for emitting an alarm signal upon sensing an alarm from said alarm unit, wherein said alarm sensor has first and second determination means for evaluating an input signal containing an alarm from the alarm unit and external noises, and the first determination means evaluates a random external noise contained in the input signal (S4, S5) whereas the second determination means evaluates a reflected sound caused by reverberations of the alarm (S6, S7) to emit an alarm signal (S9).