An image decoding method according to the present document may be characterized by including the steps of: determining whether to parse an MTS index for applying MTS to the current block; and applying the MTS to the current block on the basis of the MTS index to derive residual samples for the current block, wherein whether to parse the MTS index is determined on the basis of a flag value indicating whether a transform coefficient for a luma component of the current block is present, and the MTS index is not parsed when the flag value is 0.