Transmission power control with dynamic step value depending on a location of a mobile terminal in a radio zone is realized. The mobile terminal detects its location, whether close to a radio base station, a boundary of the radio zone, or an intermediate location, by an intensity of reception field strength of a radio signal transmitted by a radio base station. The radio base station, which has received and measured the radio signal quality transmitted by the mobile terminal, instructs the transmission power control information (increasing or decreasing) to the mobile terminal in accordance with the measured radio signal quality. When "decreasing" instructions are received repeatedly at the mobile terminal when the mobile terminal is located near the radio base station, a larger step value than a normal case is used for decreasing transmission power of the mobile terminal. When "increasing" instructions are received repeatedly at the mobile terminal when the mobile terminal is located in the boundary of the radio zone, a larger step value than the normal case is used for increasing transmission power of the mobile terminal.