A mobile terminal transmits an N-th authentication key to an authentication server when the mobile terminal has moved from a coverage area under a certain radio access point to a coverage area under another radio access point. The N-th authentication key is generated by applying a hash function to a random number a number of times one smaller than an (N-1)th authentication key which was transmitted when the mobile terminal moved to the coverage area under the certain radio access point. Upon receipt of the N-th authentication key from the mobile terminal, the authentication server applies the hash function once to the N-th authentication key, and compares the result with the (N-1)th authentication key. Then, the authentication server determines that the authentication is successful when there is a match between both keys.