A subscriber data restoring type IMS (IP Multimedia subsystem) network system includes a P-CSCF (Proxy Call Session Control Function) connected to a SIP (Session Initiation Protocol) terminal as a subscriber terminal via an access network; a S-CSCF (Serving Call Session Control Function) located within a service provision area of a provider to which the SIP terminal subscribes; and a HSS (Home Subscriber Server) storing user subscription information. The S-CSCF transmits a SIP RESET signal to the P-CSCF neighboring thereto when losing subscriber data acquired by registration processing; receives a SIP REGISTER signal transmitted by the P-CSCF in response to the SIP RESET signal; and performs re-registration processing according to the received SIP REGISTER signal to restore the lost subscriber data from the HSS.