Abstract: A cable signal detector for a connector at an end of a communication cable or for a relay connector connected with the connector includes a rectifier circuit that branches and extracts a portion of signal transmitted through the communication cable, rectifies the extracted signal into a DC signal and outputs the DC signal, a comparator that outputs a predetermined voltage signal if a voltage of the DC signal is not less than a preset threshold voltage, and a light-emitting circuit that emits light when the comparator outputs the predetermined voltage signal.