1. A method of deriving a motion vector of a bi-predictive image block by applying a bit operation, comprising: selecting a list 0 motion vector of a co-located image block in a reference picture for the bi-predictive image block as a motion vector for deriving motion vectors of the bi-predictive image block regardless of whether the co-located block includes a list 1 motion vector; and deriving at least one motion vector of the bi-predictive block by applying a bit operation to the selected motion vector.