1. A decoding method for decoding a bit stream in an image decoding apparatus, the method comprising: receiving a luma quantization parameter and a weight parameter that is added to the luma quantization parameter as the bit stream; decoding, in a decoding unit in the image decoding apparatus, the bit stream, and generating a luma component of quantized coefficients and a chroma component of quantized coefficients; and performing, in a dequantization unit in the image decoding apparatus, dequantization on the luma component of quantized coefficients using the luma quantization parameter and the chroma component of quantized coefficients using a chroma quantization parameter calculated on the basis of the luma quantization parameter weighted by an add operation of the weight parameter.