Upon receiving a frame from a transmitting radio device, a propagation attenuation calculator calculates a propagation attenuation amount from information in the received frame and registers a propagation attenuation amount for each transmission rate in a table. When transmitting a frame, a transmission power controller, selects optimum transmission power to the transmission destination radio device from the table and sends the frame at the selected optimum transmission power, and detects whether there is a value smaller than the selected optimum transmission power in the optimum transmission power at a minimum transmission rate of a radio device different from a transmission destination radio device registered in the optimum transmission power table.