1. A prediction image generation method for generating a prediction image by decoding encoded data obtained by encoding a video image having a luminance component and first and second color difference components, the method comprising: 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 first weighting factor for the luminance component, (b) a second weighting factor for the first color difference component, (c) a third weighting factor for the second color difference component, (d) a first offset for the luminance component, (e) a second offset for the first color difference component, and (f) a third offset for the second color difference component, and (2) information of motion vectors; deriving, for the to-be-decoded block, the reference images, the weighting factors and the offsets from the given number of indices; and generating a prediction image by multiplying the luminance and the first and second color difference components of the given number of reference images by the weighting factors corresponding to the respective reference images and adding the offsets, based on the motion vectors.