A generating section generates a presentation symbol string for receiving a next authentication request to an authenticated ID, a storing section stores a transformation rule and a sending destination of the presentation symbol string to be associated with each ID, a sending section sends “the generated presentation symbol string” to a display terminal, which is “the sending destination of the presentation symbol string stored in the storing section to be associated with the authenticated ID”, an updating section causes the storing section to further store the presentation symbol string sent to be associated with the authenticated ID to perform update, a receiving section receives an authentication request that designates an ID and a transformed symbol string from an access terminal, and an authenticating section causes authentication of the ID to succeed when a condition is satisfied where “the received transformed symbol string matches a transformation result obtained by applying ‘the transformation rule stored to be associated with the ID designated by the received authentication request’ to ‘the presentation symbol string stored to be associated with the ID’.”