To provide a method using a public key coding system for transferring an electronic mail between an electronic mail transmitter and an electronic mail receiver, actualizing easy transmission/reception of the electronic mail by using an ISP server instead of a public key certificate by an existing authentication station, and to provide its device.

A trigger message 3 to which user authentication data and a public key 1α are added is received from a transmission side client 1A. When the user authentication data in the trigger message 3 is authenticated, reliability is imparted to the public key 1α in the trigger message 3 to be transmitted to a reception side client. In response thereto, a response message 4 to which the user authentication data and a public key 1γ are added is received from the reception side client 1B. When the user authentication data in the response message 4 is authenticated, reliability is imparted to the public key 1γ in the response message 4 to be transmitted to the transmission side client 1A.