The effect of inherent minerals in coal on gasification in H2O and CO2 mixtures was investigated, and the synergistic effect was explained. First, low-temperature ash that has the same state as the inherent minerals in raw coal was prepared by low-temperature ashing with an oxygen plasma at 150°C. The minerals were identified by X-ray diffraction. Then, the coal was successively demineralized with deionized water, HCl, HF, and HCl/HF. Char gasification reactivities of the successively demineralized coals with different mixtures of H2O and CO2 were investigated using a thermogravimetric analyzer at 800°C. The gasification reactivity closely correlated with the Ca content in successively demineralized coals. The calcite present in coal catalyzes the gasification reaction, causing the synergistic effect.