An image decoding method performed by a decoding apparatus according to the present invention comprises: a step of deriving an intra prediction mode for a current block; a step of deriving upper neighboring samples of a plurality of rows for the current block, and left neighboring samples of a plurality of columns; a step of deriving one row of upper reference samples on the basis of the upper neighboring samples; a step of deriving one column of left reference samples on the basis of the left neighboring samples; and a step of generating a prediction sample for the current block by using at least one of the upper reference samples and the left reference samples in accordance with the intra prediction mode.