PURPOSE: To miniaturize circuit constitution by judging whether data read from the leading part of dividing areas is the leading data of a packet or not.

CONSTITUTION: A write means 14 allocates the dividing areas continuing by a number, for which the packets are stored, to the packets 1, 2,...k, all of which are detected in a detection means 10. The write means 14 writes the detection packets from the head of the allocation areas. Then, packet data are read into a judgment means 16 from the leading parts of respective dividing areas and the judging means 16 judges whether the read data is the leading data of the packets or not. When the leading data of the dividing areas in not the leading data of the packets, it is recognized that the same packet is written into plural continuous division areas including the dividing area. Thus, the circuit is miniaturized.