A method by which a decoding device decodes an image, according to the present document, comprises the steps of: receiving prediction mode information about a current block; receiving intra sub-partitions (ISP)-related information about the current block on the basis of the size and the maximum conversion block size of the current block; deriving an intra prediction mode of the current block on the basis of the prediction mode information; and generating a prediction sample of the current block on the basis of the intra prediction mode and the ISP-related information.