A relay device (1) includes a plurality of network processing units (200-1˜200-N) having the same function and structure. A session management unit (202-1) in a certain network processing unit (e.g. 200-1), when receiving a packet from an external network through an IF unit (e.g. 204-1-1) in the same processing unit, searches a session table (230-1) for IF specifying information indicative of an IF unit as an output destination determined for a session to which the above packet belongs. When the above IF specifying information indicates an IF unit (e.g. 204-N-M) in other network processing unit (e.g. 200-N), a packet processing unit (201-1) transfers the above IF specifying information and the above packet to the network processing unit (200-N) through a switch unit (300). A packet processing unit (201-N) in the network processing unit (200-N) outputs the above packet to the IF unit (204-N-M) represented by the IF specifying information.