1. An arithmetic decoding method of decoding arithmetically encoded image data formed of at least one bitplane, by using n memories that can be accessed separately, the arithmetic decoding method comprising the steps of: storing, when n-bit image data is to be decoded, n variables corresponding to pixels to be decoded, of respective n bitplanes of the n-bit image data in respective corresponding ones of the n memories; storing, when less than n-bit image data is to be decoded, one of the variables corresponding to pixels to be decoded, of each bitplane of the less than n-bit image data, in at least part of the n memories by allocating the one of the variables thereto; sequentially reading ones of the variables corresponding, respectively, to pixels to be decoded, of each bitplane, from the n memories; and sequentially decoding the pixels based on the corresponding ones of the variables sequentially read from the n memories.