To solve a problem that since a function for certifying a sender is not affixed to a mail matter itself, it is impossible to certify whether a mail matter is really sent from a sender even if the name of the sender is printed on the mail matter.

The reception control means 20 of a printer 2a receives receiver information encrypted using private key information of a sender, destination information of an original writing, and public key information of the original writing corresponding to the private key information from the sender's terminal 1a. A print control means 22 prints the encrypted receiver information, the destination information of the original writing, and the public key information of the original writing onto the envelope of mail matter Y. The receiver's decoder 3a reads out the encrypted receiver information and the public key information of the original writing printed onto the mail matter Y, and decodes the receiver information before being delivered to a display. The receiver can open the mail matter after certifying the sender by confirming the decoded receiver information of the original writing.