To easily maintain a security property while utilizing the characteristics of single sign-on within a possible range.

An SSO (single sign-on) server 10 comprises: a use log information storing part 16 for storing the using state of each user to a service server 4; a real use result information storing part 17 for storing information on the result of using the service server 4 of each user; a determining part 19 for determining to permit authentication by SSO when a use trend obtained from use log information of a logging in user coincides with the real use result of the user and when the requirement of SSO is equal to or greater than a prescribed reference value even though they are not coincident, and determines to inhibit the authentication by SSO in the other cases; and an authentication processing part 11 for performing user authentication by SSO of only a service server 4 and an application determined to be authenticateable.