A transmission power control method for controlling a transmission power of an E-DPCCH, includes: determining, at a radio network controller, that a mobile station transmitting the E-DPCCH to only a first cell should transmit the E-DPCCH to the first cell and a second cell; determining, at the radio network controller, an E-DPCCH Transmission Power Offset which is an offset from a transmission power of a DPCCH based on the determination; notifying, at the radio network controller, the E-DPCCH Transmission Power Offset to the mobile station; determining, at the mobile station, the transmission power of the E-DPCCH to be transmitted to the first cell and the second cell, based on the notified E-DPCCH Transmission Power Offset; and transmitting, at the mobile station, the E-DPCCH to the first cell and the second cell using the determined transmission power.