To obtain a mobile communication system that can perform handover without causing instantaneous interruption and packet loss and without lowering the performance of the entire network when a mobile communication system is realized by the IP network. During handover of a MN from a handover source cell to a handover destination cell, the MN obtains a CoA of a link, to which a handover destination BTS is connected, and sends it to a handover source BTS. When the handover source BTS receives the CoA from the MN, it measures a data transfer time to the handover destination BTS and transfers an IP packet destined to the MN to the handover destination BTS, and after the transfer time has passed, the handover source BTS sends it to the MN as radio data. The handover destination BTS sends the transferred data to the MN as radio data. The MN sends a message including the CoA to a CN when the handover completed. When the CN receives the message, it routes and directly sends data to the handover destination BTS.