A radio base station for use in a first mobile communication system, comprising: a radio communication unit (512, 514) configured to wirelessly communicate with a mobile station which can communicate in the first mobile communication system and in a second mobile communication system; a network communication unit (502, 504) configured to communicate with a switching station; and a determining unit configured to determine whether the mobile station can be handed over to the second mobile communication system, wherein, if a message received by the network communication unit from the switching station indicates that the mobile station is requesting a service which is not provided in the first mobile communication system but which is provided in the second mobile communication system, and if it is determined that the mobile station is provided with a handover function, and if it is determined by the determining unit that a handover to the second mobile communication system cannot be conducted, then the network communication unit requests the switching station to release a connection with the mobile station in order to conduct a redirection procedure, wherein the radio communication unit reports to the mobile station that the connection should be released in accordance with an instruction from the switching station; and wherein the determining unit is configured to determine that the handover to the second mobile communication system cannot be conducted when a radio resource for a radio access bearer for the mobile station cannot be established, and characterized in that the determining unit is further configured to determine that the handover to the second mobile communication system cannot be conducted when a CPU load of the radio base station is no less than a predetermined threshold. a radio communication unit (512, 514) configured to wirelessly communicate with a mobile station which can communicate in the first mobile communication system and in a second mobile communication system; a network communication unit (502, 504) configured to communicate with a switching station; and a determining unit configured to determine whether the mobile station can be handed over to the second mobile communication system, wherein, if a message received by the network communication unit from the switching station indicates that the mobile station is requesting a service which is not provided in the first mobile communication system but which is provided in the second mobile communication system, and if it is determined that the mobile station is provided with a handover function, and if it is determined by the determining unit that a handover to the second mobile communication system cannot be conducted, then the network communication unit requests the switching station to release a connection with the mobile station in order to conduct a redirection procedure, wherein the radio communication unit reports to the mobile station that the connection should be released in accordance with an instruction from the switching station; and wherein the determining unit is configured to determine that the handover to the second mobile communication system cannot be conducted when a radio resource for a radio access bearer for the mobile station cannot be established, and characterized in that the determining unit is further configured to determine that the handover to the second mobile communication system cannot be conducted when a CPU load of the radio base station is no less than a predetermined threshold.