An image decoding method performed by a decoding device according to the present document comprises the steps of: obtaining reference line index information and prediction information for a current block; determining an intra prediction mode of the current block on the basis of the prediction information; deriving a reference sample of the current block on the basis of the intra prediction mode and the reference line index information; and generating a prediction sample of the current block on the basis of the reference sample, wherein the intra prediction mode includes a non-directional intra prediction mode, and the reference sample is derived from among surrounding samples of the current block on the basis of the non-directional intra prediction mode and the reference line index information.