1. A motion vector coding method, comprising: an assigning step for assigning, when at least one block among a plurality of coded blocks in the neighborhood of a current block has two motion vectors which refer to reference pictures in the same direction in a display order, an identifier to each motion vector for respective one of the plurality of coded blocks; a generating step for generating a predicted motion vector for a motion vector of the current block based on the motion vectors with the same identifier among the motion vectors for the plurality of coded blocks; and a coding step for coding a difference between the motion vector of the current block and the predicted motion vector.