Abstract: This invention provides a signal break detecting circuit for reducing time required for detecting a data signal break. A difference between an average value of data signal amplified by a limiter amplifier (2) to a predetermined amplitude (output of a full-wave rectifying circuit (7)) and a peak value (output of a peak value detecting circuit (5)) is compared with a reference voltage Vth by a comparator (9) and if the difference drops below the reference voltage Vth, an alarm is dispatched. Consequently, as compared to a conventional technology in which the data signal break is detected by detecting that the peak value of the reception signal drops to a predetermined value, time required for detecting a data signal break can be reduced.