An image decoding method includes: acquiring, from a bitstream, luma weight L0 flag information indicating whether there is an L0 prediction-related weight factor and luma weight L1 flag information indicating whether there is an L1 prediction-related weight factor; determining to apply decoder-side motion vector refinement (DMVR) to an L0 motion vector and L1 motion vector for a current block, when the luma weight L0 flag information and the luma weight L1 flag information are both zero; when it has been determined to apply DMVR, deriving a refined L0 motion vector and a refined L1 motion vector by applying the DMVR to the current block; deriving prediction samples for the current block on the basis of L0 prediction using the refined L0 motion vector and L1 prediction using the refined L1 motion vector; and generating reconstruction samples for the current block on the basis of the prediction samples.