An image decoding method performed by a decoding device according to the present document comprises the steps of: deriving a motion vector for a current block; determining whether to apply refinement to the motion vector for the current block; deriving a refined motion vector by applying decoder-side motion vector refinement (DMVR) to the motion vector if it is determined to apply refinement to the motion vector for the current block; deriving prediction samples for the current block on the basis of the refined motion vector; and generating reconstruction samples for the current block on the basis of the prediction samples, wherein the step of determining whether to apply refinement includes determining whether to apply the refinement on the basis of at least one of the size of the current block and bi-prediction weight index information for the current block.