1. A video scalable encoding method used in video scalable encoding, wherein: the method calculates a weight coefficient which includes a proportional coefficient and an offset coefficient and indicates brightness variation between an encoding target image region and a reference image region in an upper layer, calculates a motion vector by applying the weight coefficient to an image signal of a reference image region as a search target and executing motion estimation, and generates a prediction signal by applying the weight coefficient to a decoded signal of a reference image region indicated by the motion vector and executing motion compensation; and the method comprises: a step that determines a data structure of the weight coefficient based on encoding information of an immediately-lower image region in an immediately-lower layer, which is present at spatially the same position as the encoding target image region; and a step that, when the immediately-lower image region performed interframe prediction in the immediately-lower layer, identifies an immediately-lower layer reference image region that the immediately-lower image region used as a prediction reference for motion prediction, and calculates the weight coefficient by applying a weight coefficient that the immediately-lower image region used in weighted motion prediction to a DC component of an image region in the upper layer, which is present at spatially the same position as the immediately-lower layer reference image region, and assuming a result of the application as a DC component of the immediately-lower image region.