A video signal decoding apparatus comprising a processor, wherein the processor is configured to: parse a syntax element related to a secondary transform of a coding unit from a bitstream of a video signal when one or more preset conditions are satisfied; check whether or not the secondary transform is applied to a transform block included in the coding unit based on the parsed syntax element; obtain one or more inverse transform coefficients for a first sub-block by performing an inverse secondary transform based on one or more coefficients of the first sub-block which is one of one or more sub-blocks constituting the transform block when the secondary transform is applied to the transform block; and obtain a residual sample for the transform block by performing an inverse primary transform based on the one or more inverse transform coefficients.