An authentication system includes: a first information processor; a second information processor; an authentication-service-providing device; and an authentication device, wherein the authentication-service-providing device has: an unit that receives second authentication information for authenticating a user from the second information processor and executes a second authentication based on the second authentication information; and an unit that issues third authentication information to the second information processor when the second authentication is successful; and the authentication device has: an unit that receives first authentication information for authenticating the user from the first information processor and executes a first authentication based on the first authentication information; and an unit that receives, from the first information processor, third authentication information obtained by the second information processor and input to the first information processor and cooperates with the authentication-service-providing device to obtain a result of a third authentication based on the third authentication information.