there is provided an optical network system in which optical signals modulated by each of at least two modulation methods are wavelength - division - multiplexed and transferred , including : an optical transmitter configured to transmit first optical signals modulated by each of at least two modulation methods ; an add - drop multiplexer configured to drop second optical signals from wavelength - division - multiplexed optical signals transferred in the optical network system , and add the first optical signals to the wavelength - division - multiplexed optical signals ; an optical receiver configured to demodulate the second optical signals corresponding to each of at least two modulation methods ; and a controller configured to control wavelengths of the first optical signals , the second optical signals and the wavelength - division - multiplexed optical signals so as to rearrange wavelengths of the first optical signals , the second optical signals and the wavelength - division - multiplexed optical signals so that optical signals modulated by a same modulation method are placed on an adjacent wavelength .