1. A motion vector coding method for coding a motion vector of a current block in a moving picture, said method comprising: a neighboring block specification step of specifying three neighboring blocks, the three neighboring blocks being located in a neighborhood of the current block; a first judgment step of judging whether or not each of the three neighboring blocks has been coded using a motion vector of another block; a second judgment step of judging whether or not a reference picture referred to in the coding of the current block is the same as a reference picture referred to in the coding of each of the three neighboring blocks; a different motion vector determining step of determining a different motion vector of each of the plurality of neighboring blocks based on a result of the first judgment step; a prediction step of deriving a predictive motion vector of the current block using different motion vectors of the plural neighboring blocks based on a result of the second judgment step; a motion vector coding step of coding a difference value between the motion vector of the current block and the predictive motion vector, wherein, in the different motion vector determining step of determining a different motion vector of each of the plurality of neighboring blocks, (1) a motion vector, which is calculated from the motion vector of the other block and which is used in performing a coding process on the neighboring block, is determined as a different motion vector of the neighboring block when it is judged in the first judgment step that the neighboring block has been coded using the motion vector of the other block, and (2) a motion vector, which is detected for the neighboring block and which is used in performing a coding process on the neighboring block, is determined as a different motion vector of the neighboring block when it is judged in the first judgment step that the neighboring block has not been coded using the motion vector of the other block, wherein, in the prediction step of deriving a predictive motion vector of the current block, (1) when it is judged in the second judgment step that three neighboring blocks refer to the same reference picture as the current block, the predictive motion vector of the current block is set to a median of three motion vectors of the three neighboring blocks, and (2) when it is judged in the second judgment step that only one of the three neighboring blocks refer to the same reference picture as the current block, the predictive motion vector of the current block is set to be a motion vector of the only one neighboring block having the same reference picture as the current block.