An image decoding method including the steps of: receiving an encode bitstream including information of I and P frames; and executing motion compensation by synthesizing a predicted image of a current frame using motion vector information included in the encoded bitstream and a reference image which is a previously decoded image, wherein said motion compensation includes calculating intensity values at points where no pixels actually exist in the reference image by interpolation, wherein said interpolation is done according to information specifying a positive rounding method or a negative rounding method included in a header section of the encoded bitstream, when the current frame is a P frame, and wherein information specifying a positive rounding method or a negative rounding method is not included in a header section of the encoded bitstream when the current frame is an I frame.