A signal detecting device includes a visualization device including a plural connectors to which a communication cable is connected, a plural detecting circuits that are provided for each of the connectors to be detected, branch and extract a part of a signal transmitted through the communication cable connected to the connectors, and indicate an existence of an data communication based on the extracted signal, and a detecting card that is insertable into and detachable from a card connector arranged in the visualization device and mounts at least a part of the detecting circuit.