1. A non-transitory computer-readable medium having stored thereon an image decoding program which, when executed by a computer, implements operations comprising: extracting motion vector information, and rounding method information from input information to be decoded; synthesizing a prediction image by performing motion compensation using the motion vector information and a reference image which is a previously decoded image; and synthesizing a decoded image by adding the prediction image to an error image; wherein a rounding method used for pixel value interpolation in performing the motion compensation for synthesizing the prediction image is one of a positive rounding method and a negative rounding method which is different from a one of a positive rounding method and a negative rounding method used for pixel value interpolation in performing the motion compensation for synthesizing a last prediction image.