User authentication method and system for implementing same

A new user authentication method which prevents illicit access to a system includes an authentication system which authenticates a user. The authentication system includes a database which manages user account information including a token ID which identifies a security token; a synchronization server which generates token codes on the basis of the token ID; and an authentication server which carries out an authentication determination transmitted from the system, and transmits the result to the system subject to use. If a prior notification of an authentication request is received prior to receiving the user authentication request, the authentication server carries out the authentication determination using a first token code. Alternatively, if the user authentication request is received without prior notification of the authentication request being received, the authentication server carries out the authentication determination using the first token code and a second token code.