1. A picture decoding apparatus which decodes a current block in a moving picture, comprising: a neighboring block specification unit operable to specify a neighboring block which is located in a neighborhood of the current block and has already been decoded; a judging unit operable to judge whether or not the neighboring block has been decoded using a motion vector calculated from a motion vector of another block located temporally after the neighboring block; a determining unit operable to determine a motion vector of the neighboring block to be an actual motion vector used in performing motion compensation on the neighboring block when the judging unit judges that the neighboring block has been decoded using the motion vector calculated from a motion vector of the other block; a motion vector prediction unit operable to derive a predictive motion vector of the current block using plural motion vectors of plural neighboring blocks; a motion vector decoding unit operable to decode a coded motion vector of the current block to obtain a decoded difference motion vector of the current block; a motion vector recovering unit operable to recover a motion vector of the current block by adding the decoded difference motion vector of the current block and the predictive motion vector; a motion compensation unit operable to generate a motion compensated picture using the recovered motion vector and a reference picture corresponding to the recovered motion vector; a picture decoding unit operable to decode a prediction error between the current block and the motion compensated picture; and a picture recovering unit operable to obtain a recovered picture 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 actual motion vector of the neighboring block is calculated from the motion vector of the other block.