A cable signal detector is configured to detect the presence or absence of information communication in a communication cable including a plurality of signal line pairs for differential signaling. The cable signal detector includes a plurality of detecting portions each of which is configured to partially branch and draw a transmission signal from a different signal line pair, rectify the branched and drawn signal into a direct current, and output that direct current, and a displaying portion configured to display the presence or absence of information communication, based on outputs of the plurality of detecting portions. The outputs of the plurality of detecting portions are connected in series, and the displaying portion is configured to display the presence or absence of information communication, based on an overall output voltage of the plurality of detecting portions connected in series.