1. A prediction image generation method for generating a prediction image by decoding encoded data obtained by encoding a video image comprising a luminance and two color differences to prediction encoding, comprising: a step of receiving an input of encoded data obtained by encoding, for a to-be-decoded block, (1) a given number of indices respectively indicating (A) a reference image and (B) a combination comprising (a) a weighting factor every luminance and two color differences and (b) an offset every luminance and two color differences, and (2) information of motion vectors; a step of deriving, for the to-be-decoded block, the reference images, the weighting factors and the offsets from the given number of indices; and a step of generating a prediction image by multiplying the given number of reference images by the weighting factors corresponding to the respective reference images and adding the given number of offsets, based on the motion vectors.