An image coding method includes the steps of storing a reference image which is a decoded image of an image previously coded based on a first prediction image synthesized by performing motion compensation, estimating motion vectors based on a comparison of an input image to be coded and the reference image, and synthesizing a second prediction image by performing motion compensation using the motion vectors and the reference image. A rounding method used for pixel value interpolation in performing the motion compensation for synthesizing the first prediction image is one of a positive rounding method and a negative rounding method, and a rounding method used for pixel value interpolation in performing the motion compensation for synthesizing the second prediction image is a different one of the positive rounding method and the negative rounding method.