Provided are a method for authenticating a user terminal in an interface server, and an interface server and a user terminal using the same. The method includes receiving authentication request information from an application service providing server in order to request the interface server to authenticate the user terminal receiving an application service provided from the application service providing server, authenticating the user terminal according to the authenticating request information using an authentication method selected by the interface server or a user of the user terminal, and transmitting authentication response information including an authentication result of performing the authentication method to the application service providing server. The interface server provides an interface for a network to the application service providing server.