A method for image decoding performed by a decoding apparatus, according to the present disclosure, comprises the steps of: obtaining residual information for a current block from a bitstream; deriving a prediction sample for the current block; deriving a residual sample for the current block on the basis of the residual information; deriving a reconstructed picture on the basis of the prediction sample and the residual sample; and performing filtering on the reconstructed picture on the basis of a convolution neural network (CNN).