A time-division multiplexing radio system controls transmission power based on an adjacent-channel leakage power ratio with precision and stability. The system has a directional coupler arranged at an output stage of a power amplifier in a transmitter and that has outputs directed to an antenna and a receiver, and a reception RF switch arranged at an input stage of a frequency converter in a receiver and that has inputs directed to the antenna and the coupler. Under transmission, the switch is switched over to the coupler so that the receiver receives the present channel to detect transmission power thereof. Also the receiver is set to receive an adjacent channel under transmission, so that adjacent-channel leakage power is detected. Based on those pieces of detected information, the ratio is calculated. To make the power ratio meet a specified condition, the transmission power of the transmitter is controlled.