A decoding apparatus for image decoding, the decoding apparatus comprising: a memory; and at least one processor connected to the memory, the at least one processor being configured to: obtain transform index information indicating transform kernels of a current block from a video signal, determine a size of a non-zero region to which an inverse primary transform is applied based on the transform kernels indicated by the transform index information and a size of the current block, wherein non-zero coefficients are located within the non-zero region, and perform the inverse primary transform on the non-zero region to which the inverse primary transform is applied by using the transform kernels indicated by the transform index information, wherein the transform kernels indicated by the transform index information include a pair of a horizontal transform kernel and a vertical transform kernel, characterized in that based on the size of the current block being 32x32 and the transform kernels applied to the current block belonging to a first transform group, the size of the non-zero region is determined as 32x32, based on the size of the current block being 32x32 and the transform kernels applied to the current block belonging to a second transform group, the size of the non-zero region is determined as 16x16, and the first transform group includes a DCT2 kernel and the second transform group does not include the DCT2 kernel. a memory; and at least one processor connected to the memory, the at least one processor being configured to: obtain transform index information indicating transform kernels of a current block from a video signal, determine a size of a non-zero region to which an inverse primary transform is applied based on the transform kernels indicated by the transform index information and a size of the current block, wherein non-zero coefficients are located within the non-zero region, and perform the inverse primary transform on the non-zero region to which the inverse primary transform is applied by using the transform kernels indicated by the transform index information, wherein the transform kernels indicated by the transform index information include a pair of a horizontal transform kernel and a vertical transform kernel, characterized in that based on the size of the current block being 32x32 and the transform kernels applied to the current block belonging to a first transform group, the size of the non-zero region is determined as 32x32, based on the size of the current block being 32x32 and the transform kernels applied to the current block belonging to a second transform group, the size of the non-zero region is determined as 16x16, and the first transform group includes a DCT2 kernel and the second transform group does not include the DCT2 kernel. obtain transform index information indicating transform kernels of a current block from a video signal, determine a size of a non-zero region to which an inverse primary transform is applied based on the transform kernels indicated by the transform index information and a size of the current block, wherein non-zero coefficients are located within the non-zero region, and perform the inverse primary transform on the non-zero region to which the inverse primary transform is applied by using the transform kernels indicated by the transform index information, wherein the transform kernels indicated by the transform index information include a pair of a horizontal transform kernel and a vertical transform kernel, characterized in that based on the size of the current block being 32x32 and the transform kernels applied to the current block belonging to a first transform group, the size of the non-zero region is determined as 32x32, based on the size of the current block being 32x32 and the transform kernels applied to the current block belonging to a second transform group, the size of the non-zero region is determined as 16x16, and the first transform group includes a DCT2 kernel and the second transform group does not include the DCT2 kernel.