The present invention relates to a station side device having a plurality of PON lines and connected thereto. A fallback mode where a partial access control unit is not allowed to communicate communication frames is performed by switching an upper switch and a lower switch in the manner shown in the following (a) and (b): (a) output destinations of downstream frames to the first and second PON lines and inputted to the upper switch are aggregated into a first access control unit, and output destinations of the downstream frames inputted from the first access control unit to the lower switch are distributed to first and second optical transmitting and receiving units and; and (b) output destinations of upstream frames inputted from the first and second optical transmitting and receiving units and to the lower switch are aggregated into the first access control unit.