1. A picture decoding method for decoding a current block in a moving picture, comprising: a neighboring block specification step of specifying a neighboring block which is located in a neighborhood of a current block and has already been decoded; a judgment step of judging if the neighboring block does not have a coded motion vector in a bit stream and has been decoded using a motion vector of another block; a different motion vector determining step of determining a different motion vector of the neighboring block based on the result of the judgment step; a prediction step of deriving a predictive motion vector of the current block using a different motion vector of the neighboring 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 motion vector 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 of the current block; a motion compensation step of generating a motion compensated picture using the recovered motion vector of the current block and a reference picture corresponding to the recovered motion vector of the current block; a decoding step of decoding a prediction error between the current block and the motion compensated picture; and, a picture recovering step of obtaining a recovered picture of the current block by adding the motion compensated picture and the decoded prediction error, wherein the other block is a block included in a current picture including the neighboring block and the current block and the other block is decoded using two motion vectors and two reference pictures corresponding to the two motion vectors, the two reference pictures comprising one forward reference picture and one backward reference picture, and when it is judged in the judgment step that the neighboring block does not have a coded motion vector in a bit stream and has been decoded using a motion vector of the other block, two motion vectors, which are calculated from motion vectors of the other block and which are used in a motion compensation of the neighboring block, are determined as two different motion vectors of the neighboring block in the different motion vector determining step, two predictive motion vectors are derived in temporally forward direction and temporally backward direction separately in the prediction step, and two motion vectors of the current block are recovered in temporally forward direction and temporally backward direction separately in the motion vector recovering step, and when it is judged in the judgment step that the neighboring block has a coded motion vector in a bit stream and has been decoded without using the motion vector of the other block, two motion vectors, which are detected in a coding process of the neighboring block and which are used in a motion compensation of the neighboring block, are determined as two different motion vectors of the neighboring block in the different motion vector determining step, two predictive motion vectors are derived in temporally forward direction and temporally backward direction separately in the prediction step, and two motion vectors of the current block are recovered in temporally forward direction and temporally backward direction separately in the motion vector recovering step.