Medium having image decoding program effecting: extracting motion vector information, quantized DCT coefficients, and rounding method information from input information; dequantizing quantized DCT coefficients to DCT coefficients; performing inverse DCT conversion on the DCT coefficients to an error image; synthesizing a prediction image of a currently decoded image by performing motion compensation using motion vector information, rounding method information, and a previously decoded reference image; and adding the prediction and error images to obtain a decoded image; wherein motion compensation is performed with half-pixel accuracy using bilinear interpolation to calculate intensity values of chrominance or luminance at points where no pixels exist in the reference image, the interpolation performable using a positive or negative rounding method, specified by rounding method information specifying one of plural values and included in coded information of the currently decoded image, in a header section of the coded information of the current image.