An image decoder wherein motion compensation is performed with half-pixel accuracy and uses bilinear interpolation to calculate intensity values of chrominance or luminance at points where no pixels actually exist in the reference image, the bilinear interpolation being performable using a positive rounding method and a negative rounding method; wherein the bilinear interpolation is performed using a rounding method specified by the rounding method information; wherein the rounding method information is included in coded information of the currently decoded image; wherein the rounding method information specifies one of two values; and wherein one of the two values specifies a positive rounding method, and another one of the two values specifies a negative rounding method.