Authentication data indicating authenticity of a client is distributed into a plurality of function data as distributed data, the plurality of function data including function data specifying a function and rule data specifying the authentication data from the function. The client and a server shares a portion of the plurality of function data. The client calculates verification data from the function data unshared with the server, by performing a calculation process difficult for a third party to calculate the function data, and transmits the verification data to the server. The server verifies authenticity of the client based on the authentication data and distributed data per client stored therein, and the verification data received from the client.