A filtering method by a decoding device according to the present invention comprises the steps of: receiving information relating to ALF control depth; deciding an ALF control unit on the basis of information relating to the division depth and the ALF control depth of a current block in a restoration picture with respect to a current picture; determining whether or not ALF is applied to the current block in an ALF control unit level; and, if the ALF is determined to be applied to the current block, performing the ALF for the current block. In the filtering method, the current picture is recursively divided on the basis of a QTBT structure, and the current block is one of blocks divided on the basis of the QTBT structure.