1. A video encoding apparatus using an inter-prediction, comprising: a video encoder configured to determine a current motion vector of a current block, predict the current block by using the determined current motion vector, and encode a residual block representing a difference between the predicted block and the current block; and a motion vector encoder configured to determine a candidate predicted motion vector set based on motion information of neighboring blocks of the current block, wherein the candidate predicted motion vector set has two candidate predicted motion vectors for the current motion vector, select a predicted motion vector of the current motion vector from among the two candidate predicted motion vectors in the candidate predicted motion vector set, and encode a differential vector representing a difference between the current motion vector and the selected predicted motion vector and a predicted motion vector index indicating the selected predicted motion vector among the two candidate predicted motion vectors.