The management server authenticates the user on the basis of the authentication request using the user name and the password received from the image forming apparatus, generates user session information obtained in a process of the user authentication process, stores the user session information in association with the IP address of the image forming apparatus in the third memory, and supplies the user session information in response to an inquiry using the IP address from the third party server. The third party server receives the authentication request using the IP address from the image forming apparatus, makes an inquiry to the management server by using the IP address, and authenticates the user on the basis of the returned user session information.