obtaining prediction mode information and residual information from a bitstream;

deriving prediction samples for a target block based on the prediction mode information;

deriving transform coefficients of the target block based on the residual information;

determining whether a non-zero transform coefficient exists in (R+1)th to Nth transform coefficients among the transform coefficients of the target block;

deriving a non-separable transform index for the target block based on the determination of whether the non-zero transform coefficient exists in the (R+1)th to Nth transform coefficients;

deriving residual samples of the target block by performing an inverse transform based on the transform coefficients of the target block, wherein the inverse transform is based on the non-separable transform index; and

generating a reconstructed picture based on the prediction samples and the residual samples,

wherein based on a case that the non-zero transform coefficient exists is included in the (R+1)th to Nth transform coefficients, a value of the non-separable transform index is derived as 0,

wherein based on a case of the value of the non-separable transform index not being equal to 0, the inverse transform based on a non-separable transform is performed on input coefficients of the inverse transform based on a transform matrix related to the non-separable transform index, and

wherein the R is a number of the input coefficients of the inverse transform, and the Nis a number of the transform coefficients in the target block.