In recent years, the introduction of a common number system has been planned by the government of Japan, and the possibility of illegal use of personal information in this system has been considered. Access records in log files are analyzed when possible illegal use is being investigated. Therefore, a method by which to maintain the integrity of the log file becomes a significant problem because alteration of digital data is very easy. Moreover, in the case of a common number system, a method that keeps verification secure as well as secret from certain organizations is needed, because various organizations will have access to the common number system. In the present paper, we propose an evidence preservation method that enables privacy and concealment to be maintained by introducing a cipher system and a hysteresis signature.