A non-transitory computer-readable storage medium storing therein a packet processing program for causing a computer to execute a process includes specifying, according to acquisition of an IP packet, on the basis of information for specifying a communication session included in a header of the acquired IP packet, a communication session in which the acquired IP packet is transmitted and received, referring to management information corresponding to the specified communication session, and discarding the acquired IP packet when the management information corresponding to an IP packet identifier of the acquired IP packet is present and, when the management information corresponding to the IP packet identifier of the acquired IP packet is absent, storing the management information corresponding to the IP packet identifier of the acquired IP packet in the storage.