To provide a WDM relay, by which noise power can be reduced, an S/N ratio is improved after light signals are transmitted, and a four optical wave mixing can be prevented from occurring even in the case a connection of made with equal interval of signal wavelength.

A WDM relay is used in a WDM transmission system where the number of qavbelengths of light signal is (n), provided, (n) is a natural number of at least 2 and equipped with an erbium-doped fiber(EDF) 1 which amplifier optical signals, a laser diode(LD) 3 which excites exciting light, a WDM coupler 2 which inputs excitation light excited by the laser diode 3 into the erbium-doped fiber 1, an optical isolator 4 connected to the output stage of the WDM coupler 2, and an optical filter 5 which is provided to the output stage of the optical isolator 4, and serves as a light transmission device which transmits only light signal of certain wavelength corresponding to the wavelength number (n) of light signals.