This invention obtains information required to quantitatively evaluate block distortion of an image on the basis of encoded data that utilizes the compression encoding technique for respective blocks. To this end, a compression encoding unit (106) compression-encodes an image input from an image input unit (104) in accordance with an encoding parameter input from an encoding parameter input unit (107). A decoding unit (108) generates a decoded image by decoding the compression-encoded data. A spatial differentiator (101) in a block distortion evaluator (100) makes a spatial derivative calculation of the original image data, and a spatial differentiator (102) makes a spatial derivative calculation of the decoded image. An evaluation information calculator (103) generates block distortion index information on the basis of these two derivative calculation results.