1. An image decoding method for decoding a bit stream in an image decoding apparatus, the method comprising: decoding, in a decoding unit in the image decoding apparatus, the bit stream and generating a chroma component of quantized coefficients; performing, in a dequantization unit in the image decoding apparatus, dequantization on the chroma component of quantized coefficients using a chroma quantization parameter calculated on the basis of a luma quantization parameter weighted by an addition operation; and performing, in a transform unit in the image decoding apparatus, an inverse orthogonal transform.