An image decoding method according to the present document comprises the steps of: receiving a quantized transform coefficient for a target block and a transform index for non-separable quadratic transform; deriving transform coefficients by inversely quantizing the quantized transform coefficient; deriving corrected transform coefficients on the basis of a transform kernel matrix in a predetermined transform set indicated by the transform index; and deriving residual samples for the target block on the basis of inverse linear transform for the corrected transform coefficients, wherein when the target block is divided into a predetermined number of sub-blocks and is coded by intra prediction, the corrected transform coefficients are derived in units of the sub-blocks, and the transform index is received for the target block.