To provide a mobile communication system capable of guaranteeing transmission sequence packets.

A source NB-RNC, when it decides to perform handover, transfers the context of a mobile unit to a target NB-RNC. When informed of completion of preparation of handover from the target NB-RNC, the mobile unit transfers a packet, which has not yet been received, to the target NB-RNC, and it directs the mobile unit to connect to the target NB-RNC. The mobile unit connects to the target NB-RNC according to the instructions from the source NB-RNC and informs the target NB-RNC of the connection. The target NB-RNC directs a core network, to switch the transfer destination of a packet from the source NB-RNC to the target NB-RNC, and starts transmission of the packets transferred from the source NB-RNC to the mobile unit.

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