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 the picture header information; a slice decoding unit to decode an image in units of slices by referring to the picture header; and an image constituting unit to generate the image in units of slices restored by the slice decoding unit as a part of an image by referring to the information on positions and sizes of the slices obtained by the slice constituting unit.