A signal transmitting device includes a transmitter, a receiver, and four optical fibers. The transmitter includes a first electro-optical converter electro-optically converting original electrical red video signal into optical red video signal in original video signal, a second electro-optical converter electro-optically converting original electrical green video signal into optical green video signal a third electro-optical converter electro-optically converting original electrical blue video signal into optical blue video signal, a parallel/serial converter converting original electrical parallel signal having a lower transfer rate than a transfer rate of the original video signal into original electrical serial signal, and a fourth electro-optical converter electro-optically converting the original electrical serial signal transmitted from the parallel/serial converter into optical serial signal. The receiver includes a first optical-electro converter, a second optical-electro converter, a third optical-electro converter, a fourth optical-electro converter, a clock signal generator, and a serial/parallel converter.