1. A motion vector coding method for coding a 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 coded; deriving a predictive motion vector of the current block using a motion vector of the neighboring block; and coding the motion vector of the current block using the predictive motion vector; wherein, when the neighboring block is coded using a motion vector of another block and said another block has two motion vectors, one of which specifies a temporally forward direction and the other of which specifies a temporally backward direction, two predictive motion vectors are derived separately in the deriving, one of which specifies a temporally forward direction and the other of which specifies a temporally backward direction, and two motion vectors of the current block are coded separately using the two predictive motion vectors in the coding of the motion vector of the current block.