To detect deterioration of an optical reception level in ONU at low lost to allow a user him/herself to perform an initial handling of a failure so that a maintenance cost produced by outsourcing to a maintenance worker can be reduced.

An ONU 1 comprises: an ONU side optical receiver 14 which receives an optical signal from an OLT 2; a deterioration level detection circuit 12 which, based on a threshold value, detects deterioration of an optical reception level; an ONU side PON controller 13 which superposes level deterioration information on data when deterioration of the optical reception level is detected and alerts an user when receiving a level deterioration alarm from the OLT 2; and an ONU side optical transmitter 15 which converts data into an optical signal and transmits it to the OLT 2. The OLT 2 comprises: an OLT side optical receiver 23 which converts each optical signal from the ONU 1 into data; an OLT side PON controller 22 which, when detecting level deterioration information from the converted data, superposes a level deterioration alarm on data; and an OLT side optical transmitter 24 which converts data into an optical signal and transmits it to the ONU 1.