<p>Provided is a mobile communication method which performs communication between a mobile station UE and a radio base station eNB by using a predetermined key generated by using a first key. The method includes: a step in which the mobile station UE updates the first key upon reception of a handover instruction signal from the radio base station eNB; a step in which the mobile station UE transmits a handover completion signal to the radio base station eNB by using a predetermined key generated by using the updated first key; and a step in which the mobile station UE performs a cell selection process and transmits a reconnection request signal to the selected reconnection destination cell by using the first key before updated if the transmission of the handover completion signal has failed and a radio link failure is detected.</p>
