Mobile communication method and radio base station

A radio base station comprising:

• a downlink data transfer unit configured to transfer downlink data to a different radio base station when it is determined that a mobile station is to perform a handover from a cell controlled by the radio base station to a cell controlled by the different radio base station, the downlink data being downlink data which are among the downlink data received from an upper level node and whose transmission to the mobile station is not completed yet;
• and
• a transfer completion notification data transmitter unit configured to transmit transfer completion notification data to the different radio base station after the transfer of the downlink data from the radio base station to the different radio base station is completed, the transfer completion notification data indicating the completion of the transfer of the downlink data,
• wherein the different radio base station is configured to perform sequence control between downlink data received from the upper level node and the downlink data received from the radio base station by use of transfer completion notification data, when the transfer completion notification data is received, and
• wherein the transfer completion notification data transmitter unit is configured to transmit the transfer completion notification data to the different radio base station, when a receiving intermediate storage at the radio base station has become empty and when no downlink data is received from the upper level node for a predetermined period of time, and
• wherein the transfer completion notification data contains no downlink data.