A radio communication apparatus and transmission power control method are disclosed, which can maintain communication with handsets while avoiding an increase in radio interference to another radio communication system as much as possible when radio communication is performed with one or more handsets. The handset transmits a communication start request signal to a base unit. Upon verifying the reception of the communication start request, the base unit measures the RSSI level of the communication start request signal transmitted from the handset, determines a transmission power value based on a power control table that has been updated with the measured RSSI level, and controls the transmission power of a control signal transmitted to the handset.