So as to provide a network service that allows a specific service to be provided and received safely for both the user and service provider, a service provider providing a user identification file that is installed in a client computer used by a user whose user eligibility is confirmed in advance. Upon request for access from the client computer, the service provider checks for the presence of a user identification file residing in the client computer in the browser screen and provides a specific service only for access requests from the client computer confirmed to have user identification files. User identification files have no effects on pre-installed programs of the client computer when they are installed and are equipped with computer identification symbols whose presence can be checked in the browser screen.