An authentication server is dynamically changed in consideration of a user's situation, a kind of service used by the user and user's convenience. When a terminal device 1 is going to receive provision of service from a service providing server 2, an authentication intermediary server 4 selects an authentication server 3 among authentication servers 3 that satisfy selection conditions previously set by the user of the terminal device 1 such as presence information, priority, usage condition, service providing server conditions and the like, so that the user of the terminal device 1 undergo authentication by the selected authentication server 3.