A data transmission system capable of mitigating the load on an uplink (feedback channel). A transmitting station of the data transmission system includes a controller for controlling packet communication quality in accordance with a CQI derived based on a packet so that the packet communication quality may become highest, a packet generator for generating a packet, and a transmitter for transmitting the packet. A receiving station of the data transmission system includes a receiver for receiving the packet, a format changer for changing a CQI format in accordance with the communication quality of the received packet, a measuring unit for measuring the CQI based on the CQI format, and a transmitter for transmitting the measured CQI to the transmitting station.