To reduce deterioration of signal quality, an optical transmission apparatus comprises a transmission means for outputting an optical signal, and a multiplexing device for outputting a wavelength multiplexed optical signal including the optical signal inputted from the transmission means, wherein the multiplexing device includes an input port that is set to transmit a first wavelength band of the optical signal inputted from the transmission means, an intensity adjustment unit that can adjust an amount of optical attenuation for each predetermined wavelength band, and adjusts the amount of optical attenuation of an edge band of the first wavelength band of the optical signal transmitted through the input port to be smaller than the amount of optical attenuation of a central band of the first wavelength band, and an output port that outputs a wavelength multiplexed optical signal including the optical signal with the adjusted amount of optical attenuation.