A terminal device for a user to use content or an application, connectable for communication to an authentication system, comprising: authentication symbol string storage means for storing an authentication symbol string; authentication symbol string generation means for generating a new authentication symbol string based on at least a part of the authentication symbol string stored in the authentication symbol string storage means, according to a generation rule for generating a new authentication symbol string based on an authentication symbol string; authentication information transmission means for sending terminal identification information for identifying the terminal device and the new authentication symbol string generated by the authentication symbol string generation means to the authentication system; means for receiving notice information sent from the authentication system in a case where it is determined in the authentication system that the new authentication symbol string sent from the authentication information transmission means belongs to a type of an authentication symbol string that is able to be generated, according to the generation rule, based on at least a part of an authentication symbol string stored in the authentication system so as to be correlated to the terminal identification information sent by the authentication information transmission means; permitting means for permitting use of the content or the application, based on the notice information; and authentication symbol string update means for updating the authentication symbol string stored in the authentication symbol string storage means to the new authentication symbol string generated by the authentication symbol string generation means in the case where it is determined in the authentication system that the new authentication symbol string sent from the authentication information transmission means belongs to the type, wherein in the case where it is determined in the authentication system that the new authentication symbol string sent from the authentication information transmission means belongs to the type, an authentication of the terminal device is successful, and the authentication symbol string stored in the authentication symbol string storage means is updated each time the authentication of the terminal device is successful. authentication symbol string storage means for storing an authentication symbol string; authentication symbol string generation means for generating a new authentication symbol string based on at least a part of the authentication symbol string stored in the authentication symbol string storage means, according to a generation rule for generating a new authentication symbol string based on an authentication symbol string; authentication information transmission means for sending terminal identification information for identifying the terminal device and the new authentication symbol string generated by the authentication symbol string generation means to the authentication system; means for receiving notice information sent from the authentication system in a case where it is determined in the authentication system that the new authentication symbol string sent from the authentication information transmission means belongs to a type of an authentication symbol string that is able to be generated, according to the generation rule, based on at least a part of an authentication symbol string stored in the authentication system so as to be correlated to the terminal identification information sent by the authentication information transmission means; permitting means for permitting use of the content or the application, based on the notice information; and authentication symbol string update means for updating the authentication symbol string stored in the authentication symbol string storage means to the new authentication symbol string generated by the authentication symbol string generation means in the case where it is determined in the authentication system that the new authentication symbol string sent from the authentication information transmission means belongs to the type, wherein in the case where it is determined in the authentication system that the new authentication symbol string sent from the authentication information transmission means belongs to the type, an authentication of the terminal device is successful, and the authentication symbol string stored in the authentication symbol string storage means is updated each time the authentication of the terminal device is successful.