Abstract: An electronic transaction in which in order to improve a reliability of message certification by digital signature and enable the use of the digital signature in a formal transaction in place of conventional signature or seal, the following procedures are implemented utilizing the fact that, in a public key cryptograph system represented by an RSA system, a first encoded message derived by encoding a first decoded message by using a public key of a first transacting party is equal to a second encoded message derived by encoding a second decoded message by using a public key of a second transacting party: (a) Check sender/receiver; (b) Add content certification function; (c) Double check the person by the possession of a secret key and the response by a terminal; (d) Add a grace period to the electronic seal; and (e) Send back a tally impression from the receiver to the sender.