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; 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; and determining that an error is included in the bitstream when the size of the image is not an integral multiple of the size of the smallest coding unit, wherein the determining includes decoding the coded image signal included in the bitstream, using an assumed size of the image, to generate a second decoded image, the assumed size being obtained under an assumption that a correct size of the image is an integral multiple of the size of the smallest coding unit.