1. An image decoder comprising a memory to store a reference image which is a previously decoded image; and a synthesizer to receive an encoded bitstream including information of images, and execute motion compensation by synthesizing a predicted image of a current frame using motion vector information included in the encoded bitstream and the reference image; wherein the synthesizing a prediction image is performable using a positive rounding method and a negative rounding method for interpolating intensity values of pixels; wherein the interpolating intensity values of pixels 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 is included in a header section of the coded information of the currently decoded image; and wherein the rounding method information specifies one of a plurality of values.