Even when the same C-RNTI is assigned to multiple mobile stations in a phantom cell, collision of the mobile stations to the same C-RNTI is assigned is avoided with simple implementation. A mobile station includes: a reception unit configured to acquire C-RNTIs set for other mobile stations UE not via a radio base station; and a transmission unit configured to give the radio base station a predetermined report when there is a mobile station UE set to have the same C-RNTI #X as a C-RNTI #X set for itself.