A MVAC spoofed frame or a loop frame generated by a user is prevented from obstructing communication and the source port ID of a fraud is reported to the network administrator. A MAC frame forwarding apparatus adds the input port ID and the source MAC address of a received frame. The MAC frame forwarding apparatus has a frame forwarding management table and a frame discarding management table. The frame discarding management table has a MAC address to be discarded, indicating a received frame to be discarded if the address matches the source MAC address of the received frame, a target port ID, and the number of discarded frames. A management apparatus discards a received frame if the source MAC address matches the MAC address to be discarded and counts the number of discarded frames. If the number of discarded frames exceeds a predetermined level, the port ID, the MAC address, and the number of discarded frames are reported to the management apparatus, and the source port of the fraud is reported to the administrator.