1. In a video scalable encoding method of the type that 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, the improvement comprising: determining a computation method for computing the proportional coefficient and the offset coefficient 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; identifying, when the immediately-lower image region performed interframe prediction in the immediately-lower layer, an immediately-lower layer reference image region that the immediately-lower image region used as a prediction reference for motion prediction; calculating 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 determining a result of the application to be a DC component of the immediately-lower image region.