A long-term signature terminal acquires storage-target data and electronic signature data corresponding to the acquired storage-target data. The acquired electronic signature data is transmitted to a server, and electronic signature data having a time stamp applied thereto is received from the server. The received electronic signature data with the applied time stamp is combined with the acquired storage-target data to generate signed data. Verification information for verifying the validity of the electronic signature data having the time stamp is received from the server and applied to the signed data to generate first long-term signature data. Confirmation data for confirming unfalsification of the generated long-term signature data is generated and transmitted to the server, and the confirmation data having a time stamp applied thereto is received from the server. The received confirmation data with the time stamp is applied to the first long-term signature data to generate second long-term signature data.