A packet information extraction circuit derives predetermined key information (for example, destination information or transmission source information) from a received packet. A HASH result computation circuit generates m (a plurality of) pseudo-random numbers (HASH results) of fixed length from the derived information. A cache table holds the key information and cumulative packet lengths within a predetermined time period by using the generated pseudo-random numbers as addresses. When a threshold value holding and comparing circuit senses that the cumulative packet length has exceeded a value held therein, it sends a notification signal to judgment means disposed in a packet exchange system. The key information items and the cumulative values of the packet lengths are stored in the cache table on the basis of the received packets in such a way that larger cumulative packet lengths are left behind, whereas smaller cumulative packet lengths are erased.