1. A picture coding method for coding a current block included in a current picture in direct mode, the picture coding 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 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; determining first and second motion vectors of the current block, using the specified one motion vector; generating a first predictive image of the current block using the first motion vector of the current block and a second predictive image of the current block using the second motion vector of the current block; generating a predictive image of the current block based on the first predictive image and the second predictive image; generating a difference image of the current block between the current block and the predictive image of the current block; and coding the difference image to obtain coded data of the current block, wherein, in the case where (i) the co-located block has two motion vectors, one 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 the other 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 (ii) the first reference picture and the second reference picture are different pictures, a motion vector among the two motion vectors of the co-located block which is output earlier in a bit stream is specified as the only one motion vector of the co-located block, in said specifying, wherein the first reference picture and the second reference picture are different pictures, and wherein the motion compensated second picture is a B-picture.