To make it possible to attain optimization of a transmission system and, at the same time, to enable efficient use of a memory space.

In a data link protocol in an SR-ARQ retransmission control system, a function to measure a transmission-rate or transmission quality of data is provided. The measured transmission quality or the transmission rate is made to change an error correction system as a transmission system of a transmission line 10. Also, a size of a frame buffer for data retransmission of the SR-ARQ is made variable by the measured transformation quality and an empty buffer part is allocated to the memory space for other processing.