An encoding device includes an encoding unit, DA converters, light sources, intensity modulators, and wavelength multiplexers. The encoding unit adds (NM/2) to an encoded signal having a negative minimum value in a range of the encoded signal among encoded signals of N channels of (NM+1) values obtained by calculating an inner product of a Hadamard matrix of N rows and N columns and a matrix having elements of N intensity signals of (M+1) values. The DA converters of the channels convert the encoded signals of the channels from digital signals into electrical analog signals. The light sources output light of wavelengths for use in the channels. The light intensity modulators of the channels intensity-modulate the light output from the light sources with the encoded signals converted into the electrical analog signals by the DA converters. The wavelength multiplexer outputs a wavelength-multiplexed signal obtained by wavelength-multiplexing the light intensity-modulated by the light intensity modulators.