Image decoder including: a memory to store a reference image which is a previously decoded 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 the 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, and 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. Such specifies one of two values specifying a positive or a negative rounding method. The rounding method information is not received from the encoded bitstream of the current frame when the current frame is an I frame.