A transmission power control method that reduces the possibility of going out of synchronization is presented. In a reduced power mode, a target SIR is set equal to or higher than SIR_reduced where deemed appropriate, whereby frequent issues of Out-of-sync and the probability of generation of RL_Failure are suppressed. To raise a reception SIR to a value equal to or higher than SIR_reduced, the target SIR may be periodically set at SIR_temp that is a value equal to or higher than SIR_reduced. Alternatively, when brought in the reduced power mode, a mobile station itself may periodically set its transmission power such that a value equal to or higher than SIR_reduced can be achieved.