1. A motion vector determining method for determining a motion vector of a current block to be coded in direct mode, with reference to a motion vector of an adjacent macroblock that is located adjacent to a current macroblock, the current block being included in the current macroblock and included in a current picture, the motion vector determining method comprising: specifying, using an adjacent macroblock specifying unit, plural adjacent macroblocks which are located adjacent to the current macroblock; specifying, using a co-located macroblock specifying unit, a co-located macroblock which is co-located with the current macroblock and included in a picture different from the current picture including the current macroblock; specifying, using a co-located block specifying unit, a co-located block which is co-located with the current block in the current macroblock and is included in the co-located macroblock, the current block and the co-located block having a size of 8 pixels×8 pixels; deriving, using a reference motion vector deriving unit, a reference motion vector of the co-located block in the case that a motion compensation of the current block in direct mode is prohibited to a size smaller than 8 pixels×8 pixels; judging, using a motion vector size judging unit, if a size of the reference motion vector is within a predetermined range; judging, using a reference picture judging unit, if a reference picture of the current block is located just after the current picture or located just before the current picture; and determining, using a motion vector determining unit, the motion vector of the current block; wherein, in the deriving of a reference motion vector, the reference motion vector of the co-located block is derived to be an actual motion vector of a corner block which is included in the co-located block and is located in a corner of the co-located macroblock, and in the determining of the motion vector of the current block, (1) the motion vector of the current block is determined to be a value of “0” when it is judged in the motion vector size judging step that the reference motion vector is within the predetermined range, and (2) the motion vector of the current block is determined to be a median of plural motion vectors of the plural adjacent macroblocks when it is judged in the motion vector size judging step that the reference motion vector is beyond the predetermined range and it is judged in the reference picture judging step that all reference pictures among a respective reference picture of the plural adjacent macroblocks are located just after the current picture or located just before the current picture.