The present invention discloses a single sign-on service system and its respective elements, and an operating method thereof. The single sign-on service system includes a terminal configured to access at least one of a plurality of application service servers according to a request for activating at least one of a plurality of applications, and receive a service token used to operate the application service from each application service servers on the basis of a single sign-on token without separately inputting sign-on information, an application service servers configured to provide data for operating the application service by the terminal having the service token and when a single sign-on message is received, provide the single sign-on message to a single sign-on service servers.