An image decoding method includes: deriving an L0 motion vector and a L1 motion vector of the current block; deriving prediction samples of the current block based on the L0 motion vector and the L1 motion vector; and generating reconstructed samples of the current block based on the prediction samples. Deriving the prediction samples includes applying bi-directional optical flow (BDOF) to the current block based on whether the condition for applying BDOF to the current block is satisfied, and the application condition of the BDOF includes a condition whereby the values of L0 luma weighted prediction flag information and L1 luma weighted prediction flag information are both zero, where the a value of each of the L0 luma weighted prediction flag information and L1 luma weighted prediction flag information being 0 represents that a weight factor for each of a L0 and L1 prediction luma components does not exist, respectively.