PURPOSE: To quantitatively grasp a block distortion generated in an image compression/expansion part by comparing the number of edges of the image for which compression and expansion are not performed and the image for which compression and expansion are performed.

CONSTITUTION: When switches S1, S2 are connected by a-c, the image data from an image data generation part 11 is processed in an image compression/ expansion part 12 and is recorded in an image memory 13. When the switches S1, S2 are connected by a-b, the image data is directly transmitted from the generation part 11 to the image memory 13 and is recorded. Next, an edge detection processing is performed by an edge detecting means for the image data recorded in the image memory 13, respectively and the number of edge as a result is calculated. By taking the difference of the number of edges, the number of edges generated due to a block distortion can be calculated and it can be defined as the reference of the evaluation of image quality.