In an image decoding method, a DC image, corresponding to each block mean value per B pixel from encoding data with respect to the HVQ system, is reproduced. A part of the DC image is made a DC nest. Image data of a target block is reproduced by synthesizing, to the DC value of target block, one or more basis vectors selected from DC nests based on the encoding data. The method is improved by setting the lowest n (n-log.sub.2 B) bits of the DC pixels in each sample to 0, and down-sampling the selected block from the DC nest and calculating the block mean value using the samples.