Image filtering method and device in image coding system

A picture filtering method performed by an encoding device, according to the present invention, comprises the steps of: deriving, from a current picture, regions for adaptive loop filtering (ALF); deciding ALF coefficients by picture unit or region unit; on the basis of the ALF coefficients, determining, by region unit, whether ALF is usable; re-deciding ALF coefficients for an ALF-usable first region; deriving a filter shape for the first region; on the basis of the re-decided ALF coefficients, determining, by unit of coding units (CUs) in the first region, whether ALF is usable; performing filtering for ALF-usable CUs on the basis of the derived filter shape and the re-decided ALF coefficients; and transmitting at least one among information on the ALF-usable first region and information on the ALF-usable CUs. According to the present invention, efficient filtering appropriate for image properties per region may be applied.