Here is provided a mobile communication system which uses a plurality of channels for communication. The plurality of channels comprise a first channel on which a mobile terminal communicates simultaneously with both a first base station and a second base station in a handover state wherein the mobile terminal is in an overlapping area of a cell of the first base station and a cell of the second base station, and a second channel on which the mobile terminal communicates with one of the first and second base stations even in the handover state. The one of the first and second base stations comprises a handover state detector 22 for detecting information notified by a radio network controller regarding whether or not the mobile terminal is in the handover state, and also comprises a transmission power calculator 20 for calculating a value of downlink transmission power on the second channel on the basis of the information detected by the handover state detector 22.