A filtering method performed by a decoding device according to the present invention comprises the steps of: deriving a specific neighboring block of a current block so as to derive filter information on the current block; deriving the specific neighboring block and a reference block within a reference picture corresponding to the specific neighboring block; deriving the filter information on the current block on the basis of the specific neighboring block and the reference block; and filtering the current block on the basis of the filter information. According to the present invention, the overall coding efficiency can be improved by deriving the filter information on the basis of the neighboring block of the already decoded current block and reducing or removing the amount of data for the transmission of the information necessary for filter generation.