1. A block distortion detection method comprising: receiving a decoded image signal obtained by decoding code sequences which are obtained by dividing an image signal into plural blocks each comprising plural pixels and then coding the image signal, and obtaining a first difference in pixel values between two pixels across the boundary of adjacent blocks, and a second difference in pixel values between two pixels which belong to a block in the vicinity of the block boundary; and performing detection of block distortion at the block boundary using the values of the first and second differences, by a detection method in which the volume of processing decreases as the resolution of the decoded image signal becomes larger.