An image decoding method according to the present document comprises a step for deriving a modified transform coefficient, wherein the step for deriving the modified transform coefficient comprises a step for determining whether or not to parse an LFNST index on the basis of whether or not the width and height of a current block satisfy a condition about whether the LFNST can be applied, and whether or not the condition about whether the LFNST can be applied is satisfied is determined on the basis of a tree type and a color format of the current block and whether or not an ISP is applied to the current block.