1. A motion vector coding method for generating a predicted motion vector for a current block to be coded and coding a motion vector of the current block using the predicted motion vector, the motion vector coding method comprising: specifying neighboring blocks which are located around the current block and have already been coded; assigning, when at least one block among the neighboring blocks has two motion vectors which refer to reference pictures in the same direction in a display order, an identifier to respective motion vector of the neighboring blocks; deriving the predicted motion vector for each motion vector of the current block by using the motion vectors having the same identifier as assigned to the each motion vector of the current block among the motion vectors of the neighboring blocks; and coding a difference motion vector between the motion vector of the current block and the predicted motion vector, wherein said deriving of the predicted motion vector includes: selecting a motion vector which refers to the same reference picture as referred to by the motion vector of the current block, from among the motion vectors of the neighboring blocks having the same identifier as the identifier assigned to the motion vector of the current block, and when the selected motion vector of the neighboring blocks is only one, deriving the one selected motion vector as the predicted motion vector of the current block.