Provided is a method that determines a reference picture index and a motion vector of a current prediction unit, derives spatial motion vector candidates using valid motion vectors of neighboring prediction units each of which exists at a predetermined position, derives temporal motion vector candidate of the current prediction unit, determines one of the spatial and temporal motions vector candidates as a motion vector predictor, calculates a motion vector difference between the motion vector of the current prediction unit and the motion vector predictor and encodes the motion vector difference and the reference picture index.Therefore, a motion vector is effectively predicted not only when motion of image is ordinary but also when the motion of image is little or steady or the image to be encoded is a background image.