Provided is an authentication system that achieves simple single sign-on used by an MFP. A management application includes a first authentication-request unit that transmits an authentication request to a management server and delivers a token to an execution platform. The other application includes a second authentication-request unit that sends an authentication request together with the token to another server. The application-execution platform includes a token-delivery unit that delivers the token to the other application. The management server includes a first authentication unit that returns a token to the first authentication-request unit, a token-creation unit, and a second authentication unit that performs authentication for an inquiry from the other server that uses a token. The other server includes a third authentication-request unit that transfers an authentication request, and transfers an authentication OK message to the second authentication-request unit.