An electronic data storage system stores electronic data with attaching an electronic signature, and output the electronic data along with the attached electronic signature, which decreases the operation costs with a simple operation. By using a public key-based signature, a third party can verify the data, and by using a secret check code, the electronic signature at registration is always valid without risk of falsification. Also by attaching an electronic signature at access, the validity of the stored data is assured, and a third party can verify the data. By using all of these features, the verification by a third party becomes possible over the long term. In this way the long term storage of electronic data is implemented.