A communication system for solving a problem, in which secure communications security cannot be ensured in communications between a femtocell base station and UE, is provided. The communication system includes UE (User Equipment) and an HLR (Home Location Register) used in an IMS (IP Multimedia subsystem) network and a femtocell base station (Femto AP) that constructs a predetermined communication area. The femtocell base station exists between the UE and the HLR, acquires a concealment key corresponding to the UE from the HLR during authentication of the UE, and executes a concealment process to conceal the identity of the UE based on the concealment key.