A group definition management system can easily acquire correspondence between an internal ID and an external ID and easily reflect information of a group definition for the internal ID to an external site. In a typical embodiment of the present invention, this is configured of an internal server 10, an external server 20, and a client. The internal server 10 has an internal user management table 15 and an internal authentication unit 11 that performs authentication by the internal ID. The external server 20 has an external user management table 24 and an external authentication unit 21 that performs authentication by the external ID and replies the information of the external ID in response to an inquiry for whether or not a specific user is an authentic user. The internal server 10 further has an authentication cooperation unit 13 that inquires the external server 20 about whether or not the user is the authentic user of the external server 20 in response to a request from the client used by the user authenticated by the internal authentication unit 11, and registers the replied external ID in the internal user management table 15 so as to correspond to the internal ID.