An authentication apparatus having a client function in a network, in which apparatus a plurality of applications (A, B) are installed for performing single sign-on to a single sign-on function providing server and thereby obtain authenticated access to service-providing servers, the authentication apparatus comprising linkage ID sharing means (Aa, Ab, Ad; Ba, Bb, Bd) configured to share a linkage ID between the applications, the linkage ID indicating a connection between the authentication apparatus including the client function and the servers (2, 10, 20) and including identification information for identifying the single sign-on function server, the single sign-on function server being configured to issue the linkage ID, and the authentication apparatus being configured to send an authentication request to the single sign-on function server, and configured to receive the linkage ID from the single sign-on function server when it has succeeded in authentication in response to the authentication request; the linkage ID sharing means comprising: a SV information management unit (Aa) configured to store the linkage ID in a SV information storing unit; an AP information management unit (Ab) configured to store connection information between the applications (A, B) in an AP information storing unit (Ae) for at least one of the plurality of applications, wherein the connection information includes an application name of an application different from the at least one of the plurality of applications and is entered by a user; and an AP decision unit (Ad) configured to determine whether the application name included in a linkage ID request received from the application different from the at least one application is stored in the AP information storing unit of the at least one application, to obtain the requested linkage ID from the SV information storing unit if the application name is stored in the AP information storing unit of the at least one application, and to return the linkage ID to the application different from the at least one application; whereby the apparatus is configured such that the at least one application logs in to the single sign-on function providing server and obtains a linkage ID and launches the application different from the at least one application which can use a service from one of the service-providing servers which the at least one application cannot use itself. a SV information management unit (Aa) configured to store the linkage ID in a SV information storing unit; an AP information management unit (Ab) configured to store connection information between the applications (A, B) in an AP information storing unit (Ae) for at least one of the plurality of applications, wherein the connection information includes an application name of an application different from the at least one of the plurality of applications and is entered by a user; and an AP decision unit (Ad) configured to determine whether the application name included in a linkage ID request received from the application different from the at least one application is stored in the AP information storing unit of the at least one application, to obtain the requested linkage ID from the SV information storing unit if the application name is stored in the AP information storing unit of the at least one application, and to return the linkage ID to the application different from the at least one application; whereby the apparatus is configured such that the at least one application logs in to the single sign-on function providing server and obtains a linkage ID and launches the application different from the at least one application which can use a service from one of the service-providing servers which the at least one application cannot use itself.