The present invention enables to reduce deterioration in the radio network capacity on the E-DPCCH, by using the transmission power offset as small as possible while satisfying a quality of the E-DPCCH in a conventional mobile communication system using "EUL". The present invention relates to a transmission power control method for controlling a transmission power of an uplink control channel (E-DPCCH) at a mobile station UE, including: measuring, at a radio base station Node B, a reception quality of a control signal received through the uplink control channel (E-DPCCH); notifying, at the radio base station Node B, the measurement result to a radio network controller RNC; determining, at the radio network controller RNC, a transmission power offset of the uplink control channel (E-DPCCH) based on the measurement result and notifying the determined transmission power offset to the mobile station UE; and controlling, at the mobile station UE, the transmission power of the uplink control channel (E-DPCCH) by using the transmission power offset.