1. A method of efficiently encoding an image, the method comprising: calculating a motion vector of a main reference image by estimating a motion of a current image with reference to the main reference image; deriving a motion vector of at least one auxiliary reference image from the calculated motion vector of the main reference image; generating a prediction image of the current image by using the motion vector of the main reference image or the motion vector of at least one auxiliary reference image; and encoding a residue image between the current image and the generated prediction image.