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 in the decoded picture header; decoding an image in units of slices by referring to the decoded picture header such that overlapping data represented in the bitstream corresponding to both of the two partially overlapping region of interest slices is used in the decoding of the image from only one of the partially overlapping region of interest slices; and generating the image in the units of the decoded slices, each unit representing a part of the image, by referring to the information on positions and sizes of the slices.