An image decoding method according to the present document comprises the steps of: obtaining intra prediction mode information and an LFNST index; deriving an intra prediction mode of a current block on the basis of intra prediction mode information; when wide-angle intra prediction is applied to the current block, deriving a modified intra prediction mode by means of intra prediction mode remapping; deciding an LFNST set comprising LFNST matrices on the basis of the modified intra prediction mode; selecting one of the LFNST matrices on the basis of the LFNST set and LFNST index; and deriving transform coefficients for the current block on the basis of the selected LFNST matrix.