An image decoding method performed by a decoding apparatus, the method comprising: deriving motion information of a first target block; deriving modified motion information for the first target block based on the motion information and bi-prediction; obtaining information on an inter prediction and residual information from a bitstream, wherein the information on the inter prediction includes mode information representing that a merge mode is applied to a second target block and a merge index information for the second target block; determining that the merge mode is applied to the second target block based on the mode information; configuring a merge candidate list for the second target block based on spatial neighboring blocks and a temporal neighboring block of the second target block; selecting one of merge candidates constituting the merge candidate list based on the merge index information for the second target block; deriving motion information of the second target block based on the selected merge candidate; generating prediction samples for the second target block by performing the inter prediction based on the motion information of the second target block; generating residual signal based on the residual information; and generating reconstructed samples based on the prediction samples and the residual signal, wherein the merge candidates include spatial motion information candidates and a temporal motion information candidate, wherein the spatial motion information candidates are derived based on the spatial neighboring blocks and the temporal motion information candidate is derived based on the temporal neighboring block, wherein when the temporal neighboring block corresponds to the first target block, the temporal motion information candidate is derived by using the modified motion information of the first target block, characterized in that when the spatial neighboring block corresponds to the first target block, the spatial motion information candidate is derived by using the motion information of the first target block. deriving motion information of a first target block; deriving modified motion information for the first target block based on the motion information and bi-prediction; obtaining information on an inter prediction and residual information from a bitstream, wherein the information on the inter prediction includes mode information representing that a merge mode is applied to a second target block and a merge index information for the second target block; determining that the merge mode is applied to the second target block based on the mode information; configuring a merge candidate list for the second target block based on spatial neighboring blocks and a temporal neighboring block of the second target block; selecting one of merge candidates constituting the merge candidate list based on the merge index information for the second target block; deriving motion information of the second target block based on the selected merge candidate; generating prediction samples for the second target block by performing the inter prediction based on the motion information of the second target block; generating residual signal based on the residual information; and generating reconstructed samples based on the prediction samples and the residual signal, wherein the merge candidates include spatial motion information candidates and a temporal motion information candidate, wherein the spatial motion information candidates are derived based on the spatial neighboring blocks and the temporal motion information candidate is derived based on the temporal neighboring block, wherein when the temporal neighboring block corresponds to the first target block, the temporal motion information candidate is derived by using the modified motion information of the first target block, characterized in that when the spatial neighboring block corresponds to the first target block, the spatial motion information candidate is derived by using the motion information of the first target block.