1. An image decoding method comprising: decoding a picture header in a bitstream; designating slices using information on positions and sizes of slices, including at least two partially overlapping region of interest slices, included the decoded picture header; decoding an image in units of slices by referring to the decoded picture header; and generating the image in units of the decoded slices as a part of an image by referring to the information on positions and sizes of the slices.