Abstract: A method for causing sending and receiving of an encrypted file between a sending user terminal and a receiving user terminal connected via a network to be performed in a secret state via a management server is provided. The sending user terminal encrypts an original file and then fragments the original file into a plurality of divided files, creates a plurality of combined files formed by combining a plurality of the divided files, and distributes and saves the combined files to which restoration information for opening the combines files has been added in a plurality of online storages. The receiving user terminal can open the combined files obtained from the online storages by using the restoration information received from the management server to extract the divided files included in the combined files, and can restore the original file from the divided files.