A method for controlling transmission power of a downlink signal which is transmitted from a base station to a mobile station depending on a base station selection code word, wherein the mobile station selects at least one primary base station among a plurality of base stations which are connected to the mobile station for soft handover to produce the base station selection code word designating said at least one primary base station, wherein an uplink signal including the base station selection code word is transmitted to the base stations, the method comprising the steps of: at each of the base stations, a) receiving the uplink signal including the base station selection code word from the mobile station; b) detecting the base station identification number, BS_IDrsv, from the base station selection code word; characterized by c) when reaching a transmission power update timing, controlling the transmission power of the downlink signal as a non-primary base station when the BS_IDrsv is not identical to an identification number of the base station which receives the base station selection code word (No 5705), otherwise controlling the transmission power of the downlink signal as the primary base station; at the mobile station, d) receiving, from said each of the base stations, the downlink signal updated based on the transmission power update timing determined so that the downlink signal received at the mobile station changes in transmission power at a predetermined timing synchronized with that of other base stations, wherein a time slot number indicating the transmission power update timing is determined by j + Tos m o d Fn, where j is a slot number in which the each of the base stations receives a last portion of the base station selection code word from the mobile station, Tos is number of slots for waiting to update the transmission power, Fn is number of slots included in one frame. at each of the base stations, a) receiving the uplink signal including the base station selection code word from the mobile station; b) detecting the base station identification number, BS_IDrsv, from the base station selection code word; characterized by c) when reaching a transmission power update timing, controlling the transmission power of the downlink signal as a non-primary base station when the BS_IDrsv is not identical to an identification number of the base station which receives the base station selection code word (No 5705), otherwise controlling the transmission power of the downlink signal as the primary base station; a) receiving the uplink signal including the base station selection code word from the mobile station; b) detecting the base station identification number, BS_IDrsv, from the base station selection code word; characterized by c) when reaching a transmission power update timing, controlling the transmission power of the downlink signal as a non-primary base station when the BS_IDrsv is not identical to an identification number of the base station which receives the base station selection code word (No 5705), otherwise controlling the transmission power of the downlink signal as the primary base station; at the mobile station, d) receiving, from said each of the base stations, the downlink signal updated based on the transmission power update timing determined so that the downlink signal received at the mobile station changes in transmission power at a predetermined timing synchronized with that of other base stations, wherein a time slot number indicating the transmission power update timing is determined by j + Tos m o d Fn, where j is a slot number in which the each of the base stations receives a last portion of the base station selection code word from the mobile station, Tos is number of slots for waiting to update the transmission power, Fn is number of slots included in one frame. d) receiving, from said each of the base stations, the downlink signal updated based on the transmission power update timing determined so that the downlink signal received at the mobile station changes in transmission power at a predetermined timing synchronized with that of other base stations, wherein a time slot number indicating the transmission power update timing is determined by j + Tos m o d Fn, where j is a slot number in which the each of the base stations receives a last portion of the base station selection code word from the mobile station, Tos is number of slots for waiting to update the transmission power, Fn is number of slots included in one frame.