1. A moving picture coding method of coding, based on inter prediction, a current block to be coded which is included in a current picture to be coded, said moving picture coding method comprising: coding the current block using a motion vector; generating a plurality of predictive motion vectors; and coding the motion vector using one of the predictive motion vectors generated in said generating, wherein when a co-located block included in a coded picture and co-located with the current block has two reference motion vectors and reference directions of the two reference motion vectors are the same, in said generating, a first prediction vector of the current block is generated using a first reference motion vector out of the two reference motion vectors, the first prediction vector and the first reference motion vector each corresponding to a first reference picture list, and a second prediction vector of the current block is generated using a second reference motion vector out of the two reference motion vectors, the second prediction vector and the second reference motion vector each corresponding to a second reference picture list.