Registered in a table are an output port number and a MAC address so as to correlate with transmission source and transmission destination IP addresses of a packet. When no information of a flow to which a received packet belongs is registered at the table, a first path unit transfers the packet to a middle path unit. When the packet from the first path unit is a packet to be a target of exception processing, the middle path unit transfers the packet to a slow path unit to cause the same to execute exception processing and otherwise, searches a routing table to obtain an output port number and a MAC address and outputs the packet to a relevant IF unit. The slow path unit registers the output port number and the MAC address obtained by the middle path unit and the transmission source and transmission destination IP addresses of the packet at the table.