1. An image decoding apparatus comprising: a picture header decoding unit to decode a picture header in a bitstream; a slice constituting unit to designate slices, including at least two partially overlapping region of interest slices, using information on positions and sizes of slices among picture header information of the decoded picture header; a slice decoding unit to decode an image in units of slices by referring to the 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 an image constituting unit to generate the image in the units of slices restored by the slice decoding unit, each unit representing a part of the image, by referring to the information on positions and sizes of the slices obtained by the slice constituting unit.