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 configured to: obtain multiple transform selection, MTS, index and residual information from a bitstream; derive transform coefficients for a current block based on the residual information; and generate residual samples of the current block based on the MTS index and the transform coefficients, wherein the MTS index represents a transform kernel set to be applied to the current block among transform kernel set candidates, wherein bins of a bin string of the MTS index are derived based on context coding, the context coding being performed using a context model represented by a context index for each of the bins for the MTS index, wherein the context index for each of the bins for the MTS index is derived as a sum of a value of context index increment for each of the bins for the MTS index and a value of context index offset for each of the bins for the MTS index, wherein the bins for the MTS index include first, second, third, and fourth bins, characterized in that the value of the context index increment for the first bin is fixed to 0, the value of the context index increment for the second bin is fixed to 1, the value of the context index increment for the third bin is fixed to 2, and the value of the context index increment for the fourth bin is fixed to 3. a memory; and at least one processor connected to the memory, the at least one processor configured to: obtain multiple transform selection, MTS, index and residual information from a bitstream; derive transform coefficients for a current block based on the residual information; and generate residual samples of the current block based on the MTS index and the transform coefficients, wherein the MTS index represents a transform kernel set to be applied to the current block among transform kernel set candidates, wherein bins of a bin string of the MTS index are derived based on context coding, the context coding being performed using a context model represented by a context index for each of the bins for the MTS index, wherein the context index for each of the bins for the MTS index is derived as a sum of a value of context index increment for each of the bins for the MTS index and a value of context index offset for each of the bins for the MTS index, wherein the bins for the MTS index include first, second, third, and fourth bins, characterized in that the value of the context index increment for the first bin is fixed to 0, the value of the context index increment for the second bin is fixed to 1, the value of the context index increment for the third bin is fixed to 2, and the value of the context index increment for the fourth bin is fixed to 3. obtain multiple transform selection, MTS, index and residual information from a bitstream; derive transform coefficients for a current block based on the residual information; and generate residual samples of the current block based on the MTS index and the transform coefficients, wherein the MTS index represents a transform kernel set to be applied to the current block among transform kernel set candidates, wherein bins of a bin string of the MTS index are derived based on context coding, the context coding being performed using a context model represented by a context index for each of the bins for the MTS index, wherein the context index for each of the bins for the MTS index is derived as a sum of a value of context index increment for each of the bins for the MTS index and a value of context index offset for each of the bins for the MTS index, wherein the bins for the MTS index include first, second, third, and fourth bins, characterized in that the value of the context index increment for the first bin is fixed to 0, the value of the context index increment for the second bin is fixed to 1, the value of the context index increment for the third bin is fixed to 2, and the value of the context index increment for the fourth bin is fixed to 3.