A moving picture decoding method for decoding a current block, comprising: determining a first merge block candidate in a merge block candidate list and a second merge block candidate in the merge block candidate list, the first merge block candidate having at least (i) a first motion vector that has been used to decode a first block neighbor to the current block, (ii) a first prediction direction corresponding to the first motion vector, and (iii) a first reference picture index value to identify a first reference picture corresponding to the first motion vector, and the second merge block candidate having at least (i) a second motion vector that has been used to decode a second block neighbor to the current block and different from the first block, (ii) a second prediction direction corresponding to the second motion vector, and (iii) a second reference picture index value to identify a second reference picture corresponding to the second motion vector, wherein the second prediction direction is different from the first prediction direction and the merge block candidate list includes a plurality of merge block candidates one of which is selected to be used for decoding the current block; generating a combined merge block candidate of bi-directional prediction by assigning the first motion vector and the first reference picture index for the first prediction direction of the combined merge block candidate and by assigning the second motion vector and the second reference picture index for the second prediction direction of the combined merge block candidate; and decoding the current block by using a merge block candidate selected from the plurality of merge block candidates including the first merge block candidate, the second merge block candidate, and the combined merge block candidate. determining a first merge block candidate in a merge block candidate list and a second merge block candidate in the merge block candidate list, the first merge block candidate having at least (i) a first motion vector that has been used to decode a first block neighbor to the current block, (ii) a first prediction direction corresponding to the first motion vector, and (iii) a first reference picture index value to identify a first reference picture corresponding to the first motion vector, and the second merge block candidate having at least (i) a second motion vector that has been used to decode a second block neighbor to the current block and different from the first block, (ii) a second prediction direction corresponding to the second motion vector, and (iii) a second reference picture index value to identify a second reference picture corresponding to the second motion vector, wherein the second prediction direction is different from the first prediction direction and the merge block candidate list includes a plurality of merge block candidates one of which is selected to be used for decoding the current block; generating a combined merge block candidate of bi-directional prediction by assigning the first motion vector and the first reference picture index for the first prediction direction of the combined merge block candidate and by assigning the second motion vector and the second reference picture index for the second prediction direction of the combined merge block candidate; and decoding the current block by using a merge block candidate selected from the plurality of merge block candidates including the first merge block candidate, the second merge block candidate, and the combined merge block candidate.