Authentication system and method by use of non-fixed user id

PURPOSE: Authentication system and method using one time identification (OTID) are provided to prevent illegal use such as illegal use of other's name using information although the user ID information is exposed. CONSTITUTION: A client terminal (100) generates n OTIDs to be used in authentication, successively select one of the n OTIDs in every authentication session and uses as a user ID. An authentication server (102) receives and stores the n OTIDs from the client terminal and inquires the selected one OTID in a database (DB) (104) according to an authentication request from the client terminal. The authentication server inspects identification between a stored secret key stored by interoperating with the viewed OTID and performs authentication. [Reference numerals] (100) Client terminal; (102) Authentication server; (AA) Internet