Presence communication system

A presence communication system capable of efficiently reducing the number of messages exchanged between presentities and watchers, thereby preventing lowering in communication quality. A presence server sets a group identifier as an identifier which is used to collectively look up a group of multiple pieces of presence information provided by presence information providers who have agreed to a use policy, aggregates and stores the presence information under the group identifier, and delivers the presence information requested to the group identifier. Each presentity provides the presence server with its presence information targeted to the group identifier. Each watcher requests the group identifier for the presence information provided by the presentities, and receives the presence information delivered from the presence server.