An image decoding method according to the present document comprises the steps of: applying LFNST to transform coefficients to derive modified transform coefficients; deriving residual samples for a target block on the basis of an inverse primary transform for the modified transform coefficients; and generating a reconstructed picture on the basis of the residual samples, wherein the step of deriving the modified transform coefficients can comprise the steps of parsing an LFNST index on the basis of the current block tree type being a dual tree chroma, and deriving the modified transform coefficients on the basis of the LFNST index and an LFNST matrix.