In using log lists of chain signatures to assure validity of an original, work of opening log lists to the public is reduced. A first signature server 2 generates a first chain signature to an original as an object of assurance, adds the generated first chain signature to a first log list, associating the first chain signature with the original, and sends a first certificate of custody including the first chain signature, to the sender of the original. Further, the first signature server 2 sends an arbitrary first chain signature in the log list to a second signature server 3, to receive a second certificate of custody. On the other hand, the second signature server 3 generates a second chain signature to the first chain signature received from the first signature server 2, adds the second chain signature to a second log list, associating the second chain signature with the first chain signature, and sends a second certificate of custody including the second chain signature, to the sender of the first chain signature. Further, the second signature server 3 sends an arbitrary second chain signature in the second log list to a public server 4.