An image decoding method comprising the steps of: receiving an encoded bitstream including information of I and P frames; determining whether a current frame of the encoded bitstream is an I frame or a P frame in accordance with frame type information included in the encoded bitstream; and executing motion compensation by synthesising a predicted image of the current frame using motion vector information included in the encoded bitstream and a reference image which is a previously decoded image, wherein the motion compensation includes pixel value interpolation for calculating intensity values at points where no pixels exist in the reference image by bilinear interpolation; characterised, only when the current frame is a P frame, by determining whether positive or negative rounding is to be used as a rounding method for the pixel value interpolation in the motion compensation, in accordance with rounding method information included in the encoded bitstream including information related to the current frame; wherein the pixel value interpolation using said positive rounding is calculated according to the equations Ib = La + Lb + 1 / 2, Ic = La + Lc + 1 / 2, and Id = La + Lb + Lc + Ld + 2 / 4, and the pixel value interpolation using said negative rounding is calculated according to the equations Ib = La + Lb / 2, Ic = La + Lc / 2, and Id = La + Lb + Lc + Ld + 1 / 4, where La, Lb, Lc, and Ld are respectively the intensity values of a first pixel, a second pixel which is horizontally adjacent to said first pixel, a third pixel which is vertically adjacent to said first pixel, and a fourth pixel which is vertically adjacent to said second pixel and horizontally adjacent to said third pixel, and Ib, Ic and Id are respectively the interpolated intensity values of the middle point between said first and second pixel, the middle point between said first and third pixel, and the middle point between said first, second, third, and fourth pixel. receiving an encoded bitstream including information of I and P frames; determining whether a current frame of the encoded bitstream is an I frame or a P frame in accordance with frame type information included in the encoded bitstream; and executing motion compensation by synthesising a predicted image of the current frame using motion vector information included in the encoded bitstream and a reference image which is a previously decoded image, wherein the motion compensation includes pixel value interpolation for calculating intensity values at points where no pixels exist in the reference image by bilinear interpolation; characterised, only when the current frame is a P frame, by determining whether positive or negative rounding is to be used as a rounding method for the pixel value interpolation in the motion compensation, in accordance with rounding method information included in the encoded bitstream including information related to the current frame; wherein the pixel value interpolation using said positive rounding is calculated according to the equations Ib = La + Lb + 1 / 2, Ic = La + Lc + 1 / 2, and Id = La + Lb + Lc + Ld + 2 / 4, and the pixel value interpolation using said negative rounding is calculated according to the equations Ib = La + Lb / 2, Ic = La + Lc / 2, and Id = La + Lb + Lc + Ld + 1 / 4, where La, Lb, Lc, and Ld are respectively the intensity values of a first pixel, a second pixel which is horizontally adjacent to said first pixel, a third pixel which is vertically adjacent to said first pixel, and a fourth pixel which is vertically adjacent to said second pixel and horizontally adjacent to said third pixel, and Ib, Ic and Id are respectively the interpolated intensity values of the middle point between said first and second pixel, the middle point between said first and third pixel, and the middle point between said first, second, third, and fourth pixel.