1. An image decoding apparatus comprising: a decoding unit configured to decode a residue of a block, from a bitstream; a weighted prediction unit configured to obtain variables for weighted prediction, including at least one multiplied value (W) and at least one additive value (O), wherein the additive value is adjusted according to a bit depth of a pixel, when the weighted prediction is applied to the block, and to obtain a predicted pixel value of the block using the weighted prediction based on the variables; and a reconstructing unit configured to reconstruct the block by using the decoded residue and the predicted pixel value, wherein the additive value is obtained based on a value of 2 (N−M) , where N represents the bit depth of the pixel and M represents a constant.