A method for decoding an image by a decoding device according to the present document comprises the steps of: receiving prediction-related information of a current block; deriving an intra prediction type of the current block on the basis of the prediction-related information; forming an intra prediction mode candidate list of the current block on the basis of intra prediction modes of blocks neighboring the current block; deriving an intra prediction mode of the current block on the basis of the intra prediction mode candidate list; and deriving reference samples of the current block on the basis of the intra prediction type and the intra prediction mode, and deriving a prediction sample on the basis of the reference samples.