1. An image information decoding method for decoding compressed image information in an image decoding apparatus, which has been coded via a process including dividing an input image signal into blocks, performing an orthogonal transform on the blocks on a block-by-block basis, and quantizing resultant orthogonal transform coefficients, said decoding process comprising: performing, in a dequantization unit in the image decoding apparatus, dequantization such that a luma quantization parameter is weighted by an addition operation that adds the weight by addition, and the dequantization is performed on a chroma component of the quantized coefficients using a chroma quantization parameter calculated on the basis of the luma quantization parameter weighted by the addition operation; and performing, in a transform unit in the image decoding apparatus, an inverse orthogonal transform.