An authentication system for performing authentication for a user who uses a usage target system, by way of using an information communication terminal, comprising: an authentication server configured to receive a user authentication request, to perform authentication determination based on the received user authentication request, and to transmit a result of the authentication determination to the usage target system; an authentication database configured to manage, for each user, user account information including a password derivation pattern constituted by specific elements selected from among elements forming a geometrical pattern and a token ID for identifying a security token of the user; and a synchronization server configured to generate, in use of a password generation algorithm, a token code based on the token ID included in the user account information and to provide the token code in time synchronization with the security token of the user to the authentication server in response to a query from the authentication server, wherein the information communication terminal and the synchronization server are provided with a security token program having the same password generation algorithm, wherein the authentication system is configured to select a first process of authentication determination in a case where an advance notice based on a start of use of the usage target system sent by the information communication terminal is received prior to receiving the user authentication request and to select a second process of authentication determination, different from the first process of authentication determination, in a case where an advance notice based on a start of use of the usage target system sent by the information communication terminal is not received prior to receiving the user authentication request. an authentication server configured to receive a user authentication request, to perform authentication determination based on the received user authentication request, and to transmit a result of the authentication determination to the usage target system; an authentication database configured to manage, for each user, user account information including a password derivation pattern constituted by specific elements selected from among elements forming a geometrical pattern and a token ID for identifying a security token of the user; and a synchronization server configured to generate, in use of a password generation algorithm, a token code based on the token ID included in the user account information and to provide the token code in time synchronization with the security token of the user to the authentication server in response to a query from the authentication server, wherein the information communication terminal and the synchronization server are provided with a security token program having the same password generation algorithm, wherein the authentication system is configured to select a first process of authentication determination in a case where an advance notice based on a start of use of the usage target system sent by the information communication terminal is received prior to receiving the user authentication request and to select a second process of authentication determination, different from the first process of authentication determination, in a case where an advance notice based on a start of use of the usage target system sent by the information communication terminal is not received prior to receiving the user authentication request.