To prepare a span list at a high speed by preparing the span list by the logical operation of the data of a reception frame sequence number list in which the sequence number of an already received frame is made correspond to the bit position of a memory and data obtained by shifting the data by one bit.

A memory 1(2) is a reception frame sequence number list, and sequence numbers {1, 4, 5} are already received. A memory 2(3) is obtained by shifting the memory 1(2) to the left by one bit, and a memory 3(4) is obtained by operating the XOR of the memory 1(2) and the memory 2(3). When the minimum sequence number is 1, and the maximum sequence number is 7, a span list (5) is obtained from the memory 3(4). Thus, the logical operation of the data of a reception frame sequence number list in which the sequence number of the already received frame is made correspond to the bit position of the memory and data obtained by shifting the data by one bit is operated.