An image decoding method performed by a decoding apparatus according to the present disclosure includes determining whether to use an advanced temporal motion vector predictor (ATMVP) candidate of sub-block unit for a current block, specifying a corresponding block corresponding to the current block in a reference picture based on a motion vector for a spatial neighboring block of the current block when the ATMVP candidate of the sub-block unit is used, deriving the ATMVP candidate of the sub-block unit for the current block based on motion vectors of sub-block unit of the corresponding block, constructing a merge candidate list including the ATMVP candidate of the sub-block unit for the current block, generating prediction samples of the current block by deriving motion vectors of the sub-block unit for the current block based on the merge candidate list and reconstructing a current picture based on the prediction samples of the current block.