In adding or deleting a radio base station PhNB managing a small cell, a transmission path of a mobile station UE is changed, while impact on a radio base station eNB and the radio base station PhNB is minimized. A mobile communication method according to the present invention includes the steps of: in a case where the mobile station UE is in communication in a macro cell and the radio base station PhNB is selected as a transmission path of user data of the mobile station UE, causing the radio base station eNB to start addition processing of adding the radio base station PhNB to the transmission path of the user data of the mobile station UE; causing the radio base station PhNB to reset header-compression related information, security setting information, and sequence-number management information in the addition processing; and causing the radio base station eNB to forward downlink user data destined for the mobile station UE, without forwarding uplink user data of the mobile station UE, to the radio base station PhNB in the addition processing.