A multicast processing section constructs, when it is determined that a received packet is a packet on a multicast packet and multicast group management protocol, a table showing a correlation between a host device and a multicast group in a port number-multicast physical address correlation storing section as well as in a multicast router-connected port storing section according to the received packet, and controls to transfer a packet for each multicast group between a multicast router and host devices according to the table.