A dynamic image transmission system comprises an input part for inputting image data of an input image, memory for storing the image data successively input from the input part and for storing transmitting image data which is to be transmitted, a detecting part for detecting each dynamic line in which corresponding picture element data of two successive frames stored in the memory differ by more than a predetermined data change quantity, a transmission part for transmitting the image data stored in the memory to a transmission path as the transmitting image data, and control part for controlling operations of the detecting part and the transmission part during an image transmission mode so that image data related to a first frame of the input image is transmitted as the transmitting image data but only the image data of each dynamic line is transmitted as the transmitting image data with respect to second and subsequent frames.