To calculate the theoretical arrival time of a cell, to adjust a cell interval and to effectively use assigned band width by deciding the theoretical arrival time of an actual cell by using a practical arrival time of an actual cell, an average cell interval, the theoretical arrival time of a preceding cell and a preceding cell interval.

This method detects an arrival time of the i-th cell (S2). It confirms (S3) whether the i is 1, and if the i is 1, the actual arrival time of the 1st cell is decided as a theoretical arrival time (S4). If the i is not 1, a preceding cell interval to the i-th cell is compared with an average cell interval (S5). If it is equal or less, it is collated whether the sum of the theoretical arrival time of the (i-1)-th cell and the average cell interval is larger than the actual arrival time of the i-th cell (S6). If it is positive, the value is decided as the theoretical arrival time of the i-th cell (S8). If it is negative, the actual arrival time of the i-th cell is decided as the theoretical arrival time (S9).