In order to complete an authentication process in shorter time in a case where there is a large number of clients which concurrently authenticate a server, the server in a server authentication system includes an address key allocation means for generating an identifier to identify each of the clients by a combination of addresses on a plurality of address spaces and allocating address keys to the respective addresses configuring the generated identifier, and a message authentication code generation means for generating message authentication codes corresponding to a message by using the address keys allocated by the address key allocation means. Each of the clients includes a server authentication means for authenticating the server based on the message authentication codes generated by the message authentication code generation means.