Relay device, signal processing device and an optical communication system

A relay apparatus including: a first interface 11 that branches an optical signal that is input in a first direction from one side of the optical transmission line, and directs the optical signal to a first path and a second path, the first path being a processing path of an optical signal having a first transmission speed, the second path being a processing path of an optical signal having a second transmission speed that is different from the first transmission speed; a processing section 12 that executes processing on an optical signal propagating through each of the paths in accordance with a corresponding transmission speed; and a second interface 13 that binds the first path and the second path of the optical signal on which the processing is executed by the processing section, by means of wavelength multiplexing and directs to the other end of the optical transmission line.