A device for deciding characteristics of a differential signal cable at a transmission rate of not less than 10 G baud includes a selector switch that changes a transmission rate or an output power of a test signal to be supplied to the differential signal cable, a signal transmitting circuit that generates a test signal at a different transmission rate or output power as one of the predetermined number of plurality of test signals according to the change of state of the selector switches and sends the generated test signal to the differential signal cable via one connector, and a signal receiving circuit that receives the test signal transmitted through the differential signal cable via the other connector and decides the success or failure of transmission of the test signal through the differential signal cable based on the received test signal and pre-stored data of the plurality of test signals.