1. In an image encoding method which comprises producing a DC image composed of each block mean value by dividing an image data per B pixel into a block, making a part of said DC image a DC nest, and where the differential vector which is obtained by separating the DC value from the pixel block to be encoded is over an allowable value, calculating one or more orthogonal basis, to which the differential vector is approximated, by the adaptive orthogonal transform using the DC nest, the improvement which comprises setting the lowest n (n&equals;log 2 B) bits of the DC pixels in each sample to 0, where the base extraction blocks are down-sampled from the DC nest and the block mean value thereof is calculated using the samples.