Provided are a plurality of fixed wavelength drop filters 35 demultiplexing optical signals having a plurality of different fixed wavelengths, a plurality of fixed wavelength add filters 36 provided corresponding respectively to the fixed wavelength drop filters 35 and adding the optical signals having the fixed wavelengths wavelength division multiplexed light flowing along a transmission path 1, a first optical branching unit 31 branching part of the wavelength division multiplexed light flowing along the transmission path 1, and a variable wavelength drop filter 32 demultiplexing the optical signal having the wavelength corresponding to a specified value from the wavelength division multiplexed light branched by the first optical branching unit 31.