Video encoding/decoding method and apparatus

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 one or more to-be-decoded blocks, a plurality of combinations comprising (A) a weighting factor every luminance and two color differences, (B) an offset every luminance and two color differences, (C) a first flag indicating presence or non-presence of the weighting factor and the offset for luminance, and (D) a second flag indicating the weighting factor and the offset for color differences and encoding, for a to-be-decoded block, an index indicating (1) information of a motion vector and (2) one combination of the plurality of combinations;
• a step of deriving, for a to-be-decoded block, the weighting factor and the offset from the index and the plurality of combinations;
• and
• a step of generating a prediction image by multiplying a reference image by the weighting factor and adding the offset, based on the motion vector.