1. An image decoding method comprising: (a) decoding a picture header in a bitstream; (b) constituting slices using information on positions and sizes of slices included the picture header decoded in (a); (c) decoding an image in units of slices by referring to the picture header decoded in (a); and (d) constituting the image in units of slices decoded in (c) as a part of an image by referring to the information on positions and sizes of the slices.