An image decoding method according to the present document may comprise a step for deriving corrected transform coefficients, wherein the step for deriving corrected transform coefficients may comprise the steps of: determining whether the tree type of the current block is a single tree; deriving a first variable and a second variable for a luma component of the current block and not deriving the first and second variables for a chroma component of the current block if the tree type is the first tree; parsing an LFNST index on the basis of the first variable and the second variable; and deriving the corrected transform coefficients by applying an LFNST kernel derived on the basis of the LFNST index.