Even when handover processing to a radio base station T-eNB not supporting “extended PDCP-SN” is performed on a mobile station UE communicating with a radio base station S-eNB supporting “extended PDCP-SN,” the communications are continued. A mobile communication method according to the invention includes the steps of: causing a radio base station S-eNB to determine to perform handover processing of a mobile station UE to a radio base station T-eNB not supporting “extended PDCP-SN” in a state where a first bearer using “extended PDCP-SN” is established with the mobile station UE, and causing the radio base station S-eNB to create “RRC connection reconfiguration” which instructs deletion of the first bearer and addition of a second bearer using “conventional PDCP-SN” and to transmit “RRC connection reconfiguration” to the mobile station UE.