The invention provides a flooding suppression method. In the case of receiving a MAC frame having a broadcast address, an unknown multicast address or an unknown unicast address as a destination address, suppression is made on the flooding which is generated by broadcasting the MAC frame to all ports other than the received port. This suppressed output control is realized by using the header information of the MAC frame, the port having received the MAC frame, a destination group identifier retrieval table (2A-T1) capable of retrieving a destination group identifier with a key of at least one combination of service VLAN identifiers corresponding to the MAC frame, and a relay port retrieval table (2A-T2) capable of retrieving either one or more relay ports to be transmitted or one or more relay ports to be untransmitted, with a key of the destination group identifier.