Image decoder including: a memory to store a previously-decoded reference image; and a synthesizer to receive an encoded bitstream including information of I and P frames, and execute motion compensation by synthesizing a predicted image of a current frame using motion vector information included in the encoded bitstream and 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. The bilinear interpolation is performed using rounding method information included in the encoded bitstream of the current frame when the current frame is a P frame, where such 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.