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; determining whether or not the neighboring block is coded using a motion vector of another block; deriving a predictive motion vector of the current block using a motion vector of the neighboring block; and coding a difference value between the motion vector of the current block and the predictive motion vector; wherein, when (i) it is determined that the neighboring block is coded using the motion vector of the other block and (ii) the neighboring block is coded using two motion vectors derived using the motion vector of the other block, one of which specifies a temporally forward direction and the other of which specifies a temporally backward direction, two predictive motion vectors of the current block are derived separately, one of which specifies a temporally forward direction and the other of which specifies a temporally backward direction, using the two motion vectors derived using the motion vector of the other block, and two difference values between two motion vectors of the current block and the two predictive motion vectors are coded separately in the coding of the difference value.