A pre-emphasis control method includes calculating an average value of transmission characteristics based on transmission characteristics of a plurality of light beams received by the receiver, and determining that, among signals of the plurality of light beams, a wavelength with a deviation from the average value is a wavelength at which control is to be performed, determining that the wavelength at which control is to be performed and a wavelength adjacent thereto are a group of wavelengths at which control is to be performed, obtaining an average of transmission characteristics of the group of wavelengths at which control is to be performed, and changing a light intensity output from each transmitter that transmits a group of wavelengths at which control is to be performed based on a difference between averaged transmission characteristics and respective transmission characteristics of the group of wavelengths at which control is to be performed.