An authentication system includes: a first terminal; a second terminal; and an authentication subsystem, wherein the first terminal transmits first identification information and a first request for push authentication to the authentication subsystem, the authentication subsystem matches the first identification information against second identification information stored in association with a unique ID of the second terminal, and if the second identification information matching the first identification information exists, transmits a push authentication operation start request to the second terminal based on the stored unique ID of the second terminal, and upon receiving the push authentication operation start request, the second terminal prompts a user to perform a predetermined operation other than inputting any of knowledge authentication information, ownership authentication information, and biometric authentication information, and when the predetermined operation is performed by the user, transmits a push authentication operation completion notification to the authentication subsystem.