1. A method of estimating an output motion vector for a transcoding, comprising: generating a group of candidate motion vectors by using a type of an output macroblock; selecting two or more reference vectors from the group of the candidate motion vectors; variably establishing a search window that varies according to a relativity among the reference vectors; and estimating the output motion vector by performing a motion vector refinement on the search window, wherein the reference vectors are composed of a first reference vector and a second reference vector, wherein the group of the candidate motion vectors is generated by grouping motion vectors of input macroblocks whose types are equal to the type of the output macroblock, and wherein selecting two or more reference vectors comprises: selecting the first reference vector by using a spatial weight and a block activity, the spatial weight being related to an overlapping region where the input macroblocks overlap with the correlation region correlated to the output macroblock, and the block activity being related to a number of non-zero DCT coefficients; and selecting the second reference vector, among the group of the candidate motion vectors excluding the first reference vector, by using the spatial weight and the block activity.