1. A picture decoding method for decoding a moving picture, comprising: a neighboring block specification step of specifying a neighboring block which is located in the neighborhood of the current block and has already been decoded; a judgment step of judging whether or not the neighboring block has been decoded using a motion vector of another block; a prediction step of deriving a predictive motion vector of the current block using a motion vector calculated from the motion vector of the other block as a motion vector of the neighboring block, when it is judged in the judgment step that the neighboring block has been decoded using the motion vector of the other block; a motion vector decoding step of decoding a coded motion vector of the current block to obtain a decoded difference motion vector of the current block; a recovering step of recovering the motion vector of the current block by adding the decoded difference motion vector of the current block and the predictive motion vector; and a decoding step of decoding the picture corresponding to the current block by using the motion vector which has been recovered in the recovering step, wherein in a case where the other block has two motion vectors, the predictive motion vector is derived in temporally forward direction and temporally backward direction separately in the prediction step, and the coded motion vector of the current block is decoded in temporally forward direction and temporally backward direction separately in the motion vector decoding step.