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 using information on positions and sizes of slices among the picture header information; a slice decoding unit to sequentially decode an image in units of slices by referring to the picture header such that each decoding of slices includes adding data of respective slices from image data corresponding to a plurality of slices to generate overlapped image data, with the decoding being based upon respective previous coding of the slices by subtracting data of respective slices from image data corresponding to the plurality of slices to remove overlapped image data from coding; 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.