Chronological identification information is composed of a plurality of cyclic numbers with priorities. For generating new chronological identification information, the chronological relation is compared in order from cyclic numbers with the highest priority to extract the newest chronological identification information in the chronological relation; when the newest chronological identification information in the chronological relation is extracted, a cyclic number with a priority as a comparison target in the extraction of the extracted chronological identification information is determined to be a cyclic number with the priority in the newly generated chronological identification information. When the cyclic numbers with a priority compared as comparison targets in the chronological identification information all are equal, no further extraction is carried out and the cyclic number subsequent to the cyclic numbers is determined to be a cyclic number with the priority in the newly generated chronological identification information.