A transmission power control method for controlling a transmission power of an E-HICH for uplink user data transmitted from a cell controlled by a radio base station to a mobile station, includes: notifying, from a radio network controller to at least one radio base station controlling a first cell and a second cell, offsets between the transmission power of the E-HICH and a transmission power of a DPCH, when a mobile station is performing a soft-handover with the first and second cell; determining, at the first cell, a transmission power of a first E-HICH based on a notified offset, and transmitting the first E-HICH to the mobile station using the determined transmission power; and determining, at the second cell, transmission power of a second E-HICH based on a notified offset, and transmitting the second E-HICH to the mobile station using the determined transmission power.