A data transmission system including a transmitter which has functions for encoding image data and transmitting it as one unit data, and a receiver which is connected to the transmitter over a communication line and has functions for sending its minimum transmission time to the transmitter and receiving one unit data from the transmitter, wherein the transmitter has a minimum transmission time detecting device for detecting the minimum transmission time of the receiver connected to the transmitter, a subtraction device for calculating deviation by subtracting the time required for transmitting each one unit data from the minimum transmission time detected by the minimum transmission time detecting device, an accumulating device for cumulatively storing deviations calculated by the subtraction device and an addition device for adding nul information to the one unit of data transmitted only when the sum of the deviations becomes less than zero.