1. A method of determining a reference picture for blocks constituting a current picture, the method comprising: (a) preparing tag information describing motion between a plurality of pictures in a reference index list; and (b) determining, by a motion estimation unit, one of the plurality of pictures to be the reference picture by referring to the tag information and performing a motion estimation process on at least one block constitutina a current picture with respect to the plurality of pictures in the reference index list; wherein another motion estimation process for the blocks constituting a current picture is performed by using the determined reference picture.