Provided is a wavelength division multiplexing (WDM) optical transmission device which is suitable of performing splitting, wavelength multiplexing, switching, and routing on an optical WDM signal in which optical signals having a spectrum close to a rectangle are arranged with a high density, and efficiency of spectral usage is ultimately high in units of wavelengths. A WDM optical transmission device according to the present invention is configured to cause flat portions of adjacent transmission bands on a wavelength spectrum overlap each other.
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