An optical transmitter includes: a light source; a wavelength detecting unit configured to detect a wavelength of light emitted from the light source; a wavelength difference information generating unit configured to generate wavelength difference information representing a wavelength difference between a predetermined reference wavelength and a wavelength of the light detected by the wavelength detecting unit; a subcarrier count determination unit configured to determine a number of subcarriers to be used, based on the wavelength difference indicated by the wavelength difference information; a multi-valued level determination unit configured to determine, for each of the subcarriers, a multi-valued level which is a number of bits transmitted by a corresponding subcarrier of the subcarriers; a modulated signal generating unit configured to generate a modulated signal by modulating the subcarriers such that each of the subcarriers has the determined multi-valued level.