The invention concerns a signal transmission device 1 including a signal sender 2 that sends first and second transmission signals S(+), S(-) of mutually opposite phases, a first transmission path 31 over which the first transmission signal S(+) is transmitted, a second transmission path 32 over which the second transmission signal S(-) is transmitted, and a signal receiver that converts the first transmission signal received from the first transmission path and the second transmission signal received from the second transmission path into an output signal Sout of a single phase. The signal transmission device differentiates each amplitude of the first and second transmission signals sent from the signal sender, and the signal receiver, based on an amplitude ratio of the first and second transmission signals, converts the received first and second transmission signals.