1. A decoding method of decoding, using a decoding apparatus having a processor, coded data with a resolution of N/M times (M and N are integers, and 1≦N<M and M>2) that of an original image, the decoding method comprising: a decoding step of receiving the coded data that are encoded by decomposing the original image into M uniform subbands using an analysis filter bank that includes M filters of the same bandwidth and M 1/M-times downsampling units, extracting N signals from decomposed signals from a low frequency side, and decoding, using the decoding apparatus, the N signals by using an entropy decoding method; and a bandwidth synthesizing step of synthesizing the N signals, that are decoded to obtain an image of the resolution of N/M times that of the original image, by using a synthesizing filter bank that includes N N-times upsampling units and N filters of the same bandwidth, wherein the coded data includes information of resolution levels defined from the low frequency side in ascending order of subband in the decomposed subbands, and the decoding step includes a calculation step of obtaining a resolution of the original image and a predetermined resolution, and calculating the value N suitable for the predetermined resolution by using the resolution of the original image and the decomposition number M.