In receiving a MAC (media access control) frame whose destination address is any of a broadcast address, an unknown multicast address and an unknown unicast address, the flooding is reduced by broadcasting the received MAC frame to all the other ports than one that has received the MAC frame. An output control that attains this object is realized by using a destination group identifier (ID) look-up table (2-T1) which enables a look-up of a destination group ID by using, as a key, any one or combination of header information of the MAC frame, a port which received the MAC frame and a service VLAN (virtual local area network) ID corresponding to the MAC frame and a backbone port look-up table (2-T2) which enables a look-up of at least one backbone port which is to transmit or at least one backbone port which is not to transmit by using the destination group ID as a key.