An in-loop filtering method for a current block performed by a video decoder, the method comprising: deriving a candidate list based on neighboring blocks to a current block; selecting a specific candidate from the candidate list; deriving (S900; S1100) at least one specific neighboring block which is the specific candidate; deriving (S910; S1110) a reference block in a reference picture indicated by a motion vector of each of the at least one specific neighboring block; deriving (S920; S1120) information about the filter for the current block based on the at least one specific neighboring block and the reference block corresponding to each of the at least one specific neighboring block; and filtering (S930; S1130) the current block based on the information about the filter, wherein the candidate list includes a combination of a left neighboring block and an upper-left neighboring block, a combination of the left neighboring block, the upper-left neighboring block, and an upper neighboring block, a combination of the left neighboring block, the upper-left neighboring block, the upper neighboring block, and an upper-right neighboring block, and a combination of the upper neighboring block and the upper-right neighboring block, wherein the information about the filter indicates at least one of a type, a size or a filter coefficient of the filter for the current block, wherein deriving the information about the filter for the current block based on the at least one specific neighboring block and the reference block corresponding to each of the at least one specific neighboring block includes: deriving filter information for the at least one specific neighboring block via a comparison between reconstructed samples of the at least one specific neighboring block and reconstructed samples of the reference block corresponding to each of the at least one specific neighboring block; and deriving the filter information for the current block based on the filter information for the at least one specific neighboring block. deriving a candidate list based on neighboring blocks to a current block; selecting a specific candidate from the candidate list; deriving (S900; S1100) at least one specific neighboring block which is the specific candidate; deriving (S910; S1110) a reference block in a reference picture indicated by a motion vector of each of the at least one specific neighboring block; deriving (S920; S1120) information about the filter for the current block based on the at least one specific neighboring block and the reference block corresponding to each of the at least one specific neighboring block; and filtering (S930; S1130) the current block based on the information about the filter, wherein the candidate list includes a combination of a left neighboring block and an upper-left neighboring block, a combination of the left neighboring block, the upper-left neighboring block, and an upper neighboring block, a combination of the left neighboring block, the upper-left neighboring block, the upper neighboring block, and an upper-right neighboring block, and a combination of the upper neighboring block and the upper-right neighboring block, wherein the information about the filter indicates at least one of a type, a size or a filter coefficient of the filter for the current block, wherein deriving the information about the filter for the current block based on the at least one specific neighboring block and the reference block corresponding to each of the at least one specific neighboring block includes: deriving filter information for the at least one specific neighboring block via a comparison between reconstructed samples of the at least one specific neighboring block and reconstructed samples of the reference block corresponding to each of the at least one specific neighboring block; and deriving the filter information for the current block based on the filter information for the at least one specific neighboring block. deriving filter information for the at least one specific neighboring block via a comparison between reconstructed samples of the at least one specific neighboring block and reconstructed samples of the reference block corresponding to each of the at least one specific neighboring block; and deriving the filter information for the current block based on the filter information for the at least one specific neighboring block.