An image decoding method according to the present invention may comprise the steps of: generating an affine mergence candidate list for a current block; specifying one of multiple affine mergence candidates contained in the affine mergence candidate list; deriving affine seed vectors of the specified mergence candidate; deriving a sub-block motion vector for a sub-block in the current block by using the affine seed vectors, wherein the sub-block is smaller in size than the current block; deriving offset data for the sub-block; and generating a sub-prediction block for the sub-block on the basis of the affine vector and the offset data.