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