An object is to perform wavelength filtering of an optical signal while preventing filter narrowing in an optical transmission apparatus. A branching unit branches a wavelength-multiplexed optical signal including an optical signal of a first wavelength into two optical signals. A wavelength selection unit blocks an optical signal of a first wavelength band including the first wavelength in the optical signal. A filter unit allows passage of an optical signal of a second wavelength band including the first wavelength in the optical signal. A multiplexing unit multiplexes and the optical signal and an optical signal of a second wavelength. The second wavelength band is wider than the first wavelength band.