A transmission power control method in a mobile satellite communication system implementing accurate control of transmission power of a base station by obtaining an average received signal level at a mobile station at high accuracy. The mobile station measures an average received signal level Q.sub.k in a predetermined period (step SP2), and informs the base station of the average received signal level Q.sub.k and an actual measuring time T.sub.k (step SP3). The base station calculates a total measuring time t.sub.k which is the sum total of the actual measuring time T.sub.k (step SP16), a corrected average received signal level R.sub.k associated with the average received signal level during the total measuring time t.sub.k (step SP18), and a measuring error .DELTA.D.sub.k (step SP17), and controls the transmission power on the basis of these values (step SP21). Estimating the measuring error .DELTA.D.sub.k in the total measuring time t.sub.k longer than the individual actual measuring time T.sub.k makes it possible to reduce the measuring error, thereby implementing highly accurate transmission power control.