1. A method for encoding a motion vector comprising: defining a candidate motion vector set with respect to a current motion vector of a current block; selecting a predicted motion vector from the defined candidate motion vector set; determining a predictability of the selected predicted motion vector in a decoding apparatus; encoding the current motion vector by using a motion vector prediction mode for indicating the selected predicted motion vector and the predictability, if the step of determining the predictability turns out positive; and encoding the current motion vector by using a motion vector prediction mode for indicating a preset default predicted motion vector and an unpredictability, if the step of determining the predictability turns out negative.