Computer-readable medium having 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 for synthesizing a prediction image of a currently decoded image; and synthesizing via motion compensation using the motion vector information and reference image, using a positive and negative rounding method for interpolating pixel intensity values; wherein interpolation of pixel intensity values uses a rounding method specified by the rounding method information included in the encoded current P frame bitstream; wherein the rounding method information is not received from the encoded I frame bitstream; wherein the rounding method information includes a header section of the currently decoded image's coded information, and specifies one of two values; and wherein one specifies a positive rounding method, and another a negative rounding method; and wherein the rounding method information is one bit.