An image decoding method performed by a decoding device according to the present document is characterized by including the steps of acquiring chroma quantization parameter data of at least one chroma quantization parameter (QP) mapping table for chroma components; deriving the chroma QP mapping table on the basis of the chroma quantization parameter data; deriving chroma quantization parameters for the chroma components on the basis of the chroma QP mapping table; deriving residual samples for the chroma components on the basis of the chroma quantization parameters; and generating a reconstructed picture on the basis of the residual samples.