PURPOSE: To confirm the picture quality of an image transmitted to the equipment of a communicating party by an operator and to reduce the mental burden of the operator by providing a means to calculate the density change of the color image, calculating the part of the large density change to easily degrade the image and executing the compression, extension and display of the part.

CONSTITUTION: A scanner part 1 inputs the color image and transmits color image data to a density change discrimination part 2. The discrimination part 2 divides the color image into the blocks of 128×128 dots, for example, calculates the density changes at the respective blocks and calculate the block having the largest density change. A compression part 3 compresses the color image data and transmits the compressed data to a communication control part 4, and the control part 4 executes the communication with the equipment of the communicating party and transmits the compressed color image data. An extension part 5 extends the compressed color image data and transmits an image corresponding to the block, which is calculated by the discrimination part 2, having the large density change to a display part 6, and the display part 6 displays the image.