A system which performs both identification and authentication of a user ID at the same time with only one step of inputting a one-time password without the need of step of inputting a user ID is provided. A one-time password deriving rule for generating a one-time password by being applied to pattern elements at specific positions in a case where a plurality of pattern elements arranged in a predetermined pattern are presented to a user who is to be authenticated as a presentation pattern is set as a password of the user, and the user is authenticated on the basis of a character string for authentication in which a user ID is embedded at a predetermined embedding position within the one-time password. In this event, a correct character string for authentication for the presented presentation pattern is generated as a verification code for all user IDs, and in a case where a verification code which matches a character string for authentication input from the user exists, user authentication of the user ID corresponding to the verification code is made successful.