In a mobile communication method according to the present invention includes the steps of: transmitting, from a handover source radio base station to a switching center, a handover request including an NCC, a PCI and a KeNB*; changing, at the switching center, the NCC, changing, at the switching center, the KeNB* on the basis of the PCI, and transmitting, from the switching center to the handover target radio base station, the handover request including the changed NCC and the changed KeNB*; generating, at the handover target radio base station, a first key on the basis of the KeNB*; and generating, at the mobile station, the first key on the basis of the NCC and the PCI included in a handover command.