Distribution processing control unit determines a main processing range and a sub-processing range whose charges are taken by a cluster member to which the unit belongs. Among packets multicast to a cluster system, a filter hands over packets matching with the main processing range and the sub-processing range to a session processing unit. The session processing unit conducts session processing (including processing of updating session information stored in a holding unit). Thereafter, when the packet matches with the main processing range, the session processing unit hands the packet over to a packet forwarding unit and when the packet matches with the sub-processing range, abandons the packet. When a cluster member in charge of main processing of a packet whose sub-processing is taken charge of by the cluster member in question develops a failure, a failure recovery unit changes the sub-processing range to the main processing range.