A plurality of management cards including an active card and a standby card are provided. The active card determines open or block of a ring port in accordance with an event based on a ring protocol, issues an open instruction or a block instruction to a line card, and notifies a block factor in addition to the block instruction when issuing the block instruction. The line card controls open or block of the ring port in accordance with the open instruction or the block instruction and retains open/block information of the ring port and a block factor of the block state in a port management table. When the standby card is changed to the active card in accordance with a predetermined change instruction, it acquires the information retained in the port management table from the line card.