Computer-readable medium with an image decoding program performing: storing a reference image of a previously decoded image; receiving coded information including motion vector and rounding method information specifying a rounding method used in synthesizing a prediction image of a currently decoded image; and synthesizing prediction image by performing motion compensation using the motion vector information and reference image; wherein synthesizing a prediction image uses a positive and negative rounding method for interpolating pixel intensity values; wherein the interpolation of pixel intensity values uses a rounding method specified by the rounding method information; wherein interpolation is via the rounding method specified by rounding method information included in the encoded bitstream when the current frame is a P frame; wherein the rounding method information is not in the encoded bitstream when the current frame is an I frame; and wherein the rounding method information specifies one of a plurality of values.