1. A bi-prediction coding method using a plurality of reference pictures, comprising the steps of: (a) selecting a first selected motion vector from a first reference picture for a current block to be coded; (b) calculating a first calculated motion vector for a second reference picture based on the first selected motion vector; (c) calculating first predicted coding cost based on the first selected motion vector, the first calculated motion vector, a first selected motion prediction block corresponding to the first selected motion vector, and a first calculated motion prediction block corresponding to the first calculated motion vector; (d) choosing a first representative selected motion vector and a first representative calculated motion vector that satisfy a first predetermined reference, and first representative predicted coding cost based on the first representative selected motion vector and the first representative calculated motion vector by repetitively performing the steps (a) to (c); (e) selecting a second selected motion vector from the second reference picture; (f) calculating a second calculated motion vector for the first reference picture based on the second selected motion vector; (g) calculating second predicted coding cost based on the second selected motion vector, the second calculated motion vector, a second selected motion prediction block corresponding to the second selected motion vector, and a second calculated motion prediction block corresponding to the second calculated motion vector; (h) choosing a second representative selected motion vector and a second representative calculated motion vector that satisfy a second predetermined reference, and second representative predicted coding cost based on the second representative selected motion vector and the second representative calculated motion vector by repetitively performing the steps (e) to (g); (i) choosing the first representative selected motion vector as a coding target motion vector and the first representative calculated motion vector as a non-coding target motion vector if the first representative predicted coding cost is smaller than the second representative predicted coding cost, and choosing the second representative selected motion vector as the coding target motion vector and the second representative calculated motion vector as the non-coding target motion vector if the second representative predicted coding cost is smaller than the first representative predicted coding cost; and (j) coding the coding target motion vector, wherein the coding target motion vector, which is meant to be coded, and non-coding target motion vector, which is not meant to be coded, are calculated with respect to the identical current block.