In the data-transmitting apparatus, the image-comparing circuit compares, when a request for updating the image displayed is made, the image data item being input, with the image data item stored in the memories, which represents the preceding frame. The circuit therefore calculates the difference between the image data items compared. If the difference thus calculated by the image-comparing circuit is smaller than a prescribed value (if No in Step S1), the transmission-region data processing circuit generates, from the difference of the image data, the data that represents that part of the image data which corresponds to a region to be transmitted, and the data that represents this region. These data items generated by the circuit are transmitted. If the difference thus calculated is equal to or greater than the prescribed value (if Yes in Step S1), the coarse-image data processing circuit generates coarse image data, which is transmitted. Next, the high-precision image data processing circuit transmits full-size image data (Step S4).