A motion vector determining method for determining a motion vector of a current block included in a current picture, said motion vector determining method comprising: specifying, when a co-located block is motion-compensated using two motion vectors and two reference pictures corresponding to the two motion vectors, only one motion vector out of the two motion vectors, the co-located block being located in a motion compensated second picture at the same position that the current block is located in the current picture, the motion compensated second picture being a picture following the current picture in display order; and determining the motion vector of the current block, using the specified one motion vector, wherein in the case where the current block is motion-compensated in direct mode, and in the case where (i) a first motion vector of the two motion vectors of the co-located block refers to a first reference picture preceding the motion compensated second picture in display order and (ii) a second motion vector of the two motion vectors of the co-located block refers to a second reference picture preceding the motion compensated second picture in display order and (iii) the first reference picture and the second reference picture are different pictures and both stored in a long-term picture buffer, the first motion vector referring to the first reference picture is specified in said specifying of only one motion vector out of the two motion vectors, and the motion vector of the current block comprises a first motion vector and a second motion vector, said first motion vector of the current block is determined to be equal to the specified motion vector and refers to the reference picture referred to by the specified motion vector, and said second motion vector of the current block is determined to be a value of 0 and refers to the motion compensated second picture. specifying, when a co-located block is motion-compensated using two motion vectors and two reference pictures corresponding to the two motion vectors, only one motion vector out of the two motion vectors, the co-located block being located in a motion compensated second picture at the same position that the current block is located in the current picture, the motion compensated second picture being a picture following the current picture in display order; and determining the motion vector of the current block, using the specified one motion vector, wherein in the case where the current block is motion-compensated in direct mode, and in the case where (i) a first motion vector of the two motion vectors of the co-located block refers to a first reference picture preceding the motion compensated second picture in display order and (ii) a second motion vector of the two motion vectors of the co-located block refers to a second reference picture preceding the motion compensated second picture in display order and (iii) the first reference picture and the second reference picture are different pictures and both stored in a long-term picture buffer, the first motion vector referring to the first reference picture is specified in said specifying of only one motion vector out of the two motion vectors, and the motion vector of the current block comprises a first motion vector and a second motion vector, said first motion vector of the current block is determined to be equal to the specified motion vector and refers to the reference picture referred to by the specified motion vector, and said second motion vector of the current block is determined to be a value of 0 and refers to the motion compensated second picture.