1. A picture decoding apparatus which decodes a current block in a moving picture, comprising: a neighboring block specification unit that specifies a plurality of neighboring blocks which are located in a neighborhood of the current block and has already been decoded; a judging unit that judges if each of the plurality of neighboring blocks 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 unit that determines a different motion vector of each of the plurality of neighboring blocks based on the result of the judgment unit; a motion vector prediction unit that derives a predictive motion vector of the current block using different motion vectors of the plurality of neighboring blocks; a motion vector decoding unit that decodes a coded motion vector of the current block to obtain a decoded difference motion vector of the current block; a motion vector recovering unit that recovers a 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 unit that generates a motion compensated picture using the recovered motion vector and a reference picture corresponding to the recovered motion vector of the current block; a picture decoding unit that decodes a prediction error between the current block and the motion compensated picture; and a picture recovering unit that obtains 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 picture different from a picture including the neighboring block and the other block is located temporally after the neighboring block, and when the judgment unit judges that the neighboring block does not have a coded motion vector in a bit stream and has been decoded using the motion vector of the other block, a motion vector, which is calculated from the motion vector of the other block and which is used in a motion compensation of the neighboring block, is determined as a different motion vector of the neighboring block by the different motion vector determining unit, and when the judgment unit judges 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, a motion vector, which is detected in a coding process of the neighboring block and which is used in a motion compensation of the neighboring block, is determined as a different motion vector of the neighboring block by the different motion vector determining unit.