On the basis of data obtained by means of analyzing coal, a first and second coal type satisfying conditions are selected, the ash melting point of the mixed coal resulting from mixing the first and second coal types is derived on the basis of a four-dimensional state diagram for SiO2—CaO—MgO-20% Al2O3, on the basis of the ash melting point of the mixed coal and the four-dimensional state diagram, an additive causing the ash melting point of the mixed coal to be at least 1400° C. at the lowest quantity when added to the mixed coal is selected from SiO2, MgO, and CaO, the addition quantity of the additive is derived, the first coal type and second coal type are mixed to result in the mixed coal, and the addition quantity of the additive is added to the mixed coal.