A multimode optical transmission system is provided which is capable of lessening the influence of multimode dispersion occurring when an optical signal is transmitted in multimode. Light sources(101-10m) output inputted electrical signals after converting them into a plurality of optical signals having different wavelengths. A wavelength multiplexing section(200) multiplexes wavelength of a plurality of optical signals outputted from the light sources(101-10m) and outputs a wavelength multiplexed signal. A multimode optical transmission line(300) transmits the wavelength multiplexed signal optically in multimode. A mode processing section(400) extracts a plurality of optical signals of such a mode as having a specific wavelength and a specific propagation constant from the wavelength multiplexed signal transmitted over the multimode optical transmission line(300). Optical receiving sections(501-50m) receive the plurality of optical signals thus extracted and convert them into electricsignals.