1. An image decoding method for decoding a bitstream, the image decoding method comprising: extracting a size of an image from the bitstream; extracting a size of a smallest coding unit from the bitstream; judging whether or not the size of the image is an integral multiple of the size of the smallest coding unit; and decoding a coded image signal included in the bitstream, using the size of the image, when the size of the image is an integral multiple of the size of the smallest coding unit, to generate a first decoded image.