1. A moving picture decoding method for performing an inter prediction process, the inter prediction process comprising: a combining step for combining blocks having similar motion vectors of motion vectors of a plurality of decoded blocks to thereby calculate a combined area; a prediction vector calculation step for calculating a prediction vector of a block to be decoded, using the motion vectors of the combined area; a motion vector calculation step for calculating a motion vector of the target block, based on the prediction vector and a difference vector included in a coded stream; a prediction image creation step for generating a prediction image using the motion vector; and a decoded image creation step for adding a difference image included in the coded stream and the prediction image to generate a decoded image.