A device including: a receiving unit receiving a packet; a packet determining unit determining whether the received packet is a head fragment packet or an intermediate or last fragment packet; a search control unit using search data registered previously for searching for packet information and search data used for identifying the same fragment of the fragment packet on the same search address; and an assembly processing unit assembling the fragment packets, wherein the search control unit extracts a fragment identifier from the head fragment packet and registers the fragment identifier as the search data on the same search address as the search address of the search data registered previously for searching for the packet information and extracts the fragment identifier of the intermediate fragment packet or the last fragment packet, makes a search in a way that uses the fragment identifier as search target data, and assembles the fragment packets.