The application relates to handover in mobile communication systems and in particular to a radio base station in the form of an HeNB of an Long Term Evolution LTE system. In such systems, the configuration is conventionally such that a Mobility Management Entity MME performs access control for a mobile station UE to a handover destination cell in a handover procedure. In a case where the "CSG (Closed Subscriber Group) ID" of the handover destination cell and the "CSG ID" of the handover source cell are different, access control by the above-described mobile switching center MME is needed, and therefore an S1 handover must be performed, rather than an X2 handover. Moreover, even in a case where the "CSG ID" of the handover destination cell and the "CSG ID" of the handover source cell are identical, then in a case where information involved in the term of validity of the "CSG ID" of the cells that the mobile station UE is able to access is not promptly synchronized and is not consistent between the mobile station UE and a network, then access control by the MME is still needed, and therefore the S1 handover must be performed, rather than the X2 handover. Therefore, the application proposes that in the handover procedure of the mobile station UE, the source HeNB determines whether or not the "CSG ID" of the handover destination cell and the "CSG ID" of the handover source cell are consistent, by comparing "CSG ID" contained in "Measurement Report" received from the mobile station UE and "CSG ID" of the subordinate cell being managed by the HeNB. In a case where it is determined that the two are consistent, a decision is made to perform an X2 handover, and in a case where is determined that the two are not consistent, then a decision is made to perform an S1 handover.