1. A motion vector decoding method for decoding a coded motion vector of a current block in a moving picture, comprising: specifying a neighboring block which is located in a neighborhood of the current block and is previously decoded; deriving a predictive motion vector of the current block; and decoding the coded motion vector of the current block using the predictive motion vector; wherein, in the deriving of the predictive motion vector of the current block, when the neighboring block is decoded based on a motion vector of another block, the predictive motion vector of the current block is derived based on the motion vector of the another block.