To provide a key authentication system capable of realizing a high security.

An encoding means possession part 2 has an encoding means. Moreover, it possesses an encoding means newly generated by an encoding means generation part 8 by replacing therewith the encoding means possessed by the encoding means possession part 2. A decoding means possession part 3 has a decoding means. Moreover, it possesses a decoding means newly generated by a decoding means generation part 9 by replacing therewith a decoding means possessed by the decoding means possession part 3. The encoding means generation part 8 newly generates an encoding means at a predetermined timing. The decoding means generation part 9 newly generates an decoding means at a predetermined timing. Thus, this arrangement realizes a higher network security than heretofore, and the objective key authentication system capable of reducing the labor needed when a user checks the invasion of a network by a third party.