1. An image decoding apparatus comprising: a picture header decoding unit which decodes a picture header in a bitstream; a slice constituting unit which constitutes slices using information on positions and sizes of slices among the picture header information; a slice decoding unit which decodes an image in units of slices by referring to the picture header; and an image constituting unit which constitutes 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.