An object of this invention is to improve transmission characteristics in WDM transmission. An optical transmitter (10) wavelength-multiplexes signal lights of wavelengths &lgr;1 through &lgr;n and outputs them onto an optical transmission line (12). Optical powers of the signal lights of the respective wavelength &lgr;1 through &lgr;n are identical or become smaller proportional to the wavelength. One repeater span of the optical transmission line (12) is composed of an optical fibers (20, 22) and an optical amplifier (24), and a gain equalizer (26) is dispersed every several repeater spans. The optical amplifier (24) in the repeater span amplifies each signal light so that optical power of a signal on the shorter wavelength side becomes smaller than that of a signal on the longer wavelength side. The gain equalizer (26) equalizes optical powers of the respective signal lights.