When a first line card receives a frame at a port, the line card refers to monitoring results by internal communicability monitoring units of other second to fourth line cards and a monitoring result by an internal communicability monitoring unit of its own, determines a first of a transmission destination from band limiting units of other line cards based on the monitoring results, and transmits the frame. The second line card limits the communication band of the frame by the first band limiting unit of its own, and transmits the frame to a predetermined port. If any fault is detected in the communicability in the internal communicability monitoring table to/from the second line card as the communication counterpart, and, if the communicability to/from the third line card as the communication counterpart is normal, the first line card changes the transmission destination into the second band limiting unit.