Abstract: Synchronization of presence information is established between different presence services. Presence information of each user is stored on the presence server 27 of the presence display system comprising mobile phones. Presence information of the SIP phone system comprising the SIP phone 29 and the SIPG/W (enterprise side) 30 is stored in the SIPG/W (enterprise side) 30. When presence information of the presence server 27 has changed, the presence server 27 notifies the SIPG/W (enterprise side) 30 of the change in presence information via the SIPG/W (mobile communication network side) 31. Conversely, when presence information of the SIP phone system has changed, the SIPG/W (enterprise side) 30 notifies the presence server 27 of the change in presence via the SIPG/W (mobile communication network side) 31.