To improve communication efficiency by adjusting the communication rate and the length of a frame to be transmitted corresponding to the quality of a communication line.

A communication quality detecting circuit 3a calculates the error rate of a frame inputted through an input terminal 2. A fragment length control circuit 3b calculates a fragment length optimum for the current communication line corresponding to the error rate and outputs a fragment length determine signal to a frame generating circuit 4. a communication rate control circuit 3c calculates the communication rate optimum for the present communication line corresponding to the error rate and outputs a communication rate determine signal to a modem 5. A frame is inputted through an input terminal 1 to the frame generating circuit 4. When the quality of the communication line is degraded, the communication rate is decelerated rather than present one and the length of the frame to be transmitted later is shortened. When the quality of the communication line is made satisfactory, the communication rate is accelerated rather than present one and the length of the frame to be transmitted later is extended.