1. A picture decoding method for decoding a bit stream corresponding to coded data of a decoding target block included in a decoding target picture in direct mode, wherein the bit stream is generated by the following steps: a step of specifying, when a co-located block has two motion vectors, only one motion vector out of the two motion vectors, the co-located block being located in a second picture at the same position that a coding target block is located in a coding target picture, the second picture being a B-picture following the coding target picture in display order; a step of determining a first motion vector and a second motion vector of the coding target block, using the specified one motion vector; a step of generating a first predictive image of the coding target block using the first motion vector of the coding target block and a second predictive image of the coding target block using the second motion vector of the coding target block; a step of generating a predictive image of the coding target block based on the first predictive image and the second predictive image; a step of generating a difference image between the coding target block and the predictive image of the coding target block; and a step of coding the difference image to obtain a coded difference image of the coding target block, the picture decoding method comprising: specifying, when a co-located block has two motion vectors, only one motion vector out of the two motion vectors, the co-located block being located in a second picture at the same position that the decoding target block is located in the decoding target picture, the second picture being a B-picture following the decoding target picture in display order; determining a first motion vector and a second motion vector of the decoding target block, using the specified one motion vector; generating a first predictive image of the decoding target block using the first motion vector of the decoding target block and a second predictive image of the decoding target block using the second motion vector of the decoding target block; generating a predictive image of the decoding target block based on the first predictive image and the second predictive image; decoding a coded difference image of the decoding target block to obtain a decoded difference image of the decoding target block; and reconstructing the decoding target block by adding the decoded difference image of the decoding target block and the predictive image of the decoding target block, wherein, in the case where the co-located block is motion compensated using two motion vectors which refer to a same one reference picture preceding the second picture in display order, one motion vector among the two motion vectors of the co-located block which appears earlier in a bit stream is specified as the only one motion vector of the co-located block in said specifying.