Protection methods and optical communication system

Protection method according to the present invention, ONU is upon detection of a failure of OSU to themselves and logical connections, while switching the receiving wavelength to a spare wavelength when pre OSU and logical connections specified in advance for each ONU, do not delete the connection information of the OLT. The OLT, upon detecting the OSU failure by the failed OSU and ONU preliminary OSU that was logical connection takes over the information of the ONU, the failed OSU and the ONU that was logical connection to communicate in a short time resume.