1. A video decoding method, comprising: extracting a merge flag and a merge index from a bitstream; when the merge flag indicates a merge mode, checking for whether predetermined candidate adding conditions are satisfied, and adding into a merge candidate set including the merge candidates corresponding to neighboring blocks of a current block, a combined bi-predictive motion parameter, the combined bi-predictive motion parameter generated by combining a list 0 motion parameter of a first available candidate block and a list 1 motion parameter of a second available candidate block, when the predetermined candidate adding conditions are satisfied; and generating a predicted block of the current block by using a motion parameter corresponding to a merge candidate selected from the merge candidate set, the merge candidate selected by the extracted merge index, wherein the merge candidate set is generated based on a maximum number of the merge candidates, the maximum number of the merge candidates decided using information on the number of the merge candidates included in the bitstream.