In a control frame processing unit (14), the number of control frames from a frame demultiplexing unit (13) is counted for each of LLIDs given to the control frames. If the number of frames during a predetermined count period is equal to or smaller than a preset threshold, some or all of the control frames are written in a storage device (30) as processing data. When the number of frames has exceeded the threshold, writing data of the control frames having the LLID in the storage device is stopped. Hence, even when the number of control frames from ONUs increases, software processing can be executed properly.