A device for reducing ash adhesion in a boiler, includes: a calculator (9) which, in the case of mixing one or more types of solid fuels, calculates and determines the mixing ratio of the solid fuels so that the resultant solid fuel mixture has a slag viscosity not lower than a reference value at a predetermined atmospheric temperature; and fuel supply amount regulators (3a, 3b) which regulate the amount of the solid fuels to be supplied to the boiler based on the mixing ratio as described above. By using this device, ash adhesion in a boiler can be easily and accurately predicted and, therefore, ash adhesion can be reduced even in the case of using various types of solid fuels including a low-grade coal as fuel.