An intra prediction method performed by a decoding device, according to the present invention, comprises the steps of: deriving an intra prediction mode for a current block; deriving neighboring samples of the current block; and generating a prediction sample for the current block by using at least one of the neighboring samples according to the intra prediction mode, wherein the derived neighboring samples include left neighboring samples, upper left neighboring samples, upper neighboring samples, right neighboring samples, lower right neighboring samples, and lower neighboring samples of the current block. According to the present invention, intra prediction performance can be improved by using expanded neighboring reference samples, and coding efficiency for CUs can be improved.