In order to stably operate a boiler using several kinds of solid fuels including depleted ash as fuels, adhesion of ash is suppressed. A calculator (9) preliminarily collects properties of a solid fuel, such as the content rate of ash and the composition of an ash constituent, as data (8). The calculator (9) uses the mix ratio of solid fuels as a parameter and calculates the composition of an ash constituent of the mixed fuels on the basis of the preliminarily measured composition of the ash constituent of each solid fuel. The calculator (9) determines a reference value of the rate of slug by which the ash deposition ratio is reduced on the basis of the relationship between the preliminarily measured ash deposition ratio and the slag ratio. Further, the calculator (9) calculates the mix ratio of each solid fuel using a thermodynamic equilibrium calculation so as to obtain an ash composition in which the slag ratio is not more than the determined reference value. On the basis of the mix ratio of each solid fuel calculated by the calculator (9), the amount of solid fuel dispensed from hoppers (1, 2) is adjusted by a fuel supply amount adjusting device (3). Thus, each solid fuel, the dispensed amount of which has been adjusted, is mixed by a mixer (4) and crushed by a crusher (5) before being supplied to a boiler (7) as a fuel and burned by a burner (6).