In quantitative analysis by the X-ray diffraction method, it is necessary to accurately determine the integrated intensities of peaks corresponding to each component. However, it is difficult to calculate integrated intensities for a compound having low crystallinity and, therefore, broad peaks. Our new quantitative analysis method, the Direct Derivation (DD) method, performs Whole Powder Pattern Fitting (WPPF) based on the measured profiles of a single component. As a result, WPPF can be easily performed even on compounds that show complex patterns that conventionally make it difficult to calculate the profile function from the crystal structure. Here, mixtures of α-Al₂O₃ and γ-Al₂O₃ were quantified.