A transmission power control technique allowing stable and reliable signal transmission in soft handover is disclosed. Each of the base stations involved in soft handover measures an amount of loss of the base station selection signal. When the amount of loss of the base station selection signal exceeds a threshold, the transmission power of the downlink signal is not set to the minimum level but to the normally controlled level. Further, a transmission power update timing of each base station is 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.