an image decoding method is utilized to obtain high quality images without error accumulation . such an image decoding method comprises receiving an encoded bitstream including information of p and b frames , and executing motion compensation by synthesizing a predicted image of a current frame using motion vector information included in the encoded bitstream and a reference image which is a previously decoded image . the motion compensation includes calculating intensity values at points where no pixels actually exist in the reference image by interpolation performed according to information specifying a positive rounding method or a negative rounding method when the current frame is a p frame , and using a predetermined rounding method which is a positive rounding method or a negative rounding method when the current frame is a b frame .