A mail gateway generates an identifier for each attached file included in the e-mail addressed to a user and held in a mail server, and notifies a mail terminal belonging to the user of a list of generated IDs. When the user selects attached files that the user wants to attach to the e-mail from the list of IDs that is notified from the mail gateway, the mail gateway constructs e-mail including the selected attached files and the mail server transmits the constructed e-mail to addresses of the receivers of the e-mail.