1. An image decoding method comprising: determining which of an intra mode, an inter mode, or a weighted prediction mode a pixel value of a block of interest of a current image has been encoded in; restoring a predicted pixel value of the block of interest using at least one block spatially adjacent to the block of interest, among blocks included in a restored current image, upon the determining indicating that the pixel value of the block of interest has been encoded in the intra mode; restoring the predicted pixel value of the block of interest using the block of interest and blocks included in at least one restored previous image, upon the determining indicating that the pixel value of the block of interest has been encoded in the inter mode; and adaptively adjusting one or more existing weighting factors according to a number of bits used to express a pixel of the current image, and restoring the predicted pixel value of the block of interest using the one or more adjusted weighting factors, upon the determining indicating that the pixel value of the block of interest has been encoded in the weighted prediction mode, wherein the one or more existing weighting factors are adjusted using the equation 2 N-M , where N represents the number of bits, and M represents a constant.