1. A decoding method for decoding a bit stream corresponding to coded data of a decoding target block included in a decoding target picture, wherein the bit stream is generated by using the following steps: a step of specifying a co-located block which is a block included in a second picture that is different from a coding target picture, the co-located block being located in the second picture at the same position that the coding target block is located in a coding target picture; a step of determining first and second motion vectors of the coding target block for performing motion compensation on the coding target block, using a third motion vector which is a motion vector of the co-located block; 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 of the coding target block between the coding target block and the predictive image of the coding target block; and a step of coding the difference image of the coding target block to obtain coded data of the coding target block, the decoding method comprising the steps of: specifying a co-located block which is a block included in a second picture that is different from the decoding target picture, the co-located block being located in the second picture at the same position that the decoding target block is located in the decoding target picture; determining first and second motion vectors of the decoding target block for performing motion compensation on the decoding target block, using a third motion vector which is a motion vector of the co-located block; 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 the coded data of the decoding target block to obtain a difference image of the decoding target block; and reconstructing the decoding target block by adding the difference image of the decoding target block and the predictive image of the decoding target block, wherein, in the case where a reference picture referred to by the third motion vector is stored in a long-term picture buffer, (i) the first motion vector, which refers to the reference picture stored in the long term picture buffer, is determined to be equal to the third motion vector, and (ii) the second motion vector, which refers to the second picture that is different from the decoding target picture, is determined to be equal to a value of 0.