To largely reduce time for searching received messages having a reception key coincident to a reception key designated by a reception function in comparison with a conventional case.

In accordance with a hash value generated by a hash value generating means 190 from a reception key which an acquired received message has, one reception buffer is selected from among a plurality of reception buffers 161 to 164 and stored and, in response to a transmission request from a receiving means 20 for performing reception according to coincidence determination of the reception key designated by the reception function, more than one candidate of received messages in accordance with the reception functions from the reception buffers 161 to 164 related to the hash value are output. Thus, prior to the received message having the reception key coincident with the reception key designated by the reception function, a time spent searching the received message having the reception key coincident with the reception key designated by the reception function can be largely reduced in comparison with the conventional case even when a large amount of received messages having a reception key which is not coincident arrive.