Image decoding method including: receiving an encoded 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 previously-decoded reference image. The motion compensation includes calculating intensity values at points where no pixels actually exist by bilinear interpolation performable using a positive and negative rounding method; wherein the bilinear interpolation is performed using the rounding method specified by rounding method information included in the encoded bitstream of the current frame when the current frame is a P frame; wherein the rounding method information is included in a header section of the coded information of the current frame and specifies one of a plurality of values. The rounding method information is not received from the encoded bitstream of the current frame when the current frame is an I frame.