Abstract: An authenticity assurance apparatus for e-mail documents which preserves a transmitted e-mail includes a unit to add a digital signature to an e-mail document and a file attached to it at time of transmitting the mail from a sender and from the apparatus; a unit to check for a mail tampering by using the digital signature at time of receiving the mail by the apparatus and by a recipient; a unit to inform the sender and the recipient of the tampering when detected; a unit to preserve the mail and the associated data on an unoverwritable database; a unit to meet a requirement of integrity by creating and adding a time stamp; a unit to encrypt and preserve the e-mail document and the attached file; and a unit to meet a requirement of confidentiality of the e-mail document by limiting an access to the database.