Provided is an OLT including a wavelength splitter that splits all upstream optical signals with different wavelengths received from at least one ONU according to wavelength, at least one receiver that receive the optical signals split by the wavelength splitter according to wavelength, a wavelength selector that allocates an upstream transmission wavelength of the at least one ONU, and a transmitter that transmits wavelength allocation information allocated by the wavelength selector to the at least one ONU.