When retransmission requests are issued based on the results of error-correction decoding of received data packets to perform data transmission, statistical information indicating whether received data packets have been successfully decoded or have not been decoded is acquired, and based on the rate at which correct decoding has been made or the rate at which correct decoding has not been made, which is obtained from the statistical information, a request is transmitted to the transmission source to change the window size, which is the number of data units handled in a predetermined layer, so that when for example the communication environment is not excellent, processing can be performed at a small window size, and the occurrence of circumstances in which data retransmission and the discarding of received data are repeated can be efficiently prevented. Accordingly, data transmission efficiency in a wireless transmission system performing high-speed data transmission is improved under conditions of a poor radio wave environment.