A system and method to prevent collision between mobile stations to which the same C-RNTI is allocated in a simple implementation while avoiding a problem of C-RNTI depletion in a phantom cell is disclosed. A radio base station includes an allocation unit configured to allocate multiple C-RNTIs to each of mobile stations, and the allocation unit performs adjustment such that collision of search spaces to which PDCCH resources are allocated does not occur among the multiple mobile stations.