PURPOSE: To detect a falsification of the contents recorded in a recording medium, by recording a part of an information in a recording medium which cannot be rewritten.

CONSTITUTION: A main storage medium 3 stores all of the transaction contents, an auxiliary storage medium 4 and 5 save a part of the recording contents of the medium 3, and they belong to transaction persons A, B, respectively. In case A and B execute a transaction through a communication circuit 6, the transaction contents are recorded in the medium 3, and simultaneously, a part of its contents is written in unrewritable mediums 4, 5. Accordingly, in case A has falsified the contents of the medium 3, and on the contrary, in case B has falsified the contents, its fact can be detected by using the medium 5 belonging to B, respectively.