To suppress the delay time of an image frame at a receiver side by allowing a transmitter side to use the number of frames in a transmission buffer analogizing an arrival time of image frames so as to control a quantization step.

Compression data are divided into a prescribed length of frames by a frame configuration means 2, an error check bit is calculated and added to the frame and stored in a transmission buffer 3. The buffer 3 increases a frame count by one for each one frame storage and the count is given to a quantization step controller 4 as number of resident frames in the buffer 3. A receiver 200 uses a communication line error detection means 8 to detect error in received compression image data. When no error is detected, a reception confirmation signal is sent as a reception confirmation notice. When any error is detected, a re-transmission request signal is sent and frames whose error is not detected by the means 8 are stored in a reception buffer 9.