An apparatus of decoding a motion vector, the apparatus comprising: a merge candidate generator configured to determine whether motion vectors of spatial prediction units that are spatially collocated to a current prediction unit and motion vectors of temporal prediction units that are temporally collocated to the current prediction unit are available and to determine the available motion vectors as motion vector predictor candidates; characterized by: when the number of the motion vector predictor candidates is less than a predetermined number 'n', where n is an integer, the merge candidate generator generates at least one combined bi-directional motion vector by combining an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 0 and an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 1; adds the at least one combined bi-directional motion vector to the motion vector predictor candidates; and when the number of the motion vector predictor candidates is less than the predetermined number 'n', adds a motion vector having a predetermined component value to the motion vector predictor candidates so that a total number of motion vector predictor candidates is the predetermined number n, wherein the predetermined component value is (0, 0), the apparatus further comprising a motion vector decoder configured to obtain information indicating a motion vector predictor among the motion vector predictor candidates from a bitstream and to obtain the motion vector predictor of the current prediction unit based on the information. a merge candidate generator configured to determine whether motion vectors of spatial prediction units that are spatially collocated to a current prediction unit and motion vectors of temporal prediction units that are temporally collocated to the current prediction unit are available and to determine the available motion vectors as motion vector predictor candidates; characterized by: when the number of the motion vector predictor candidates is less than a predetermined number 'n', where n is an integer, the merge candidate generator generates at least one combined bi-directional motion vector by combining an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 0 and an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 1; adds the at least one combined bi-directional motion vector to the motion vector predictor candidates; and when the number of the motion vector predictor candidates is less than the predetermined number 'n', adds a motion vector having a predetermined component value to the motion vector predictor candidates so that a total number of motion vector predictor candidates is the predetermined number n, wherein the predetermined component value is (0, 0), the apparatus further comprising a motion vector decoder configured to obtain information indicating a motion vector predictor among the motion vector predictor candidates from a bitstream and to obtain the motion vector predictor of the current prediction unit based on the information. when the number of the motion vector predictor candidates is less than a predetermined number 'n', where n is an integer, the merge candidate generator generates at least one combined bi-directional motion vector by combining an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 0 and an available motion vector, from among the available motion vectors, which indicates a block of picture in reference picture list 1; adds the at least one combined bi-directional motion vector to the motion vector predictor candidates; and when the number of the motion vector predictor candidates is less than the predetermined number 'n', adds a motion vector having a predetermined component value to the motion vector predictor candidates so that a total number of motion vector predictor candidates is the predetermined number n, wherein the predetermined component value is (0, 0), the apparatus further comprising a motion vector decoder configured to obtain information indicating a motion vector predictor among the motion vector predictor candidates from a bitstream and to obtain the motion vector predictor of the current prediction unit based on the information.