A service
server can: associate identification tags which identify users, with other-user identification tags, and store said identification tags; and associate the identification tags with identification data uniquely identifying users in service servers, and an
encryption key for identification data and store said identification tags. A control
server device stores a table for storing
encryption data, encrypted identifiers of service servers used by users. The gateway
server device receives an other-user identification tag associated with an identification tag and stored in a service server. If the other-user identification tag is associated and stored, the
encryption key, the encryption data, which is associated with identification data related to the other-user identification tag and stored in the control server device, is decoded, the service server that can be used by users of the other-user identification tags is obtained; and is determined whether the first identification tag should be associated with the other-user identification tag.