According to the present disclosure, an image decoding method performed by a decoding device comprises the steps of: deriving a first intra prediction mode of a current block; deriving neighboring samples including left neighboring samples and upper neighboring samples of the current block; deriving a second intra prediction mode of a target sample on the basis of a location of the target sample and the first intra prediction mode of the current block; and performing prediction for the target sample on the basis of the second intra prediction mode of the target sample.