PURPOSE: To contrive to suppress incombustible components in a boiler and to improve dust collection efficiency by an electric dust collector, by impregnating or blending a solid fuel with a combustion improver obtained by adding an iron complex ion-forming substance to an aqueous solution of an iron salt and then neutralizing the solution.

CONSTITUTION: An iron complex ion-forming substance[e.g., citric acid, tartaric acid, sodium acetate, oxalic acid, triethanolamine, ethylenediaminetetraacetic acid (most preferably citric acid), etc.,] is added to an aqueous solution of iron salt and the solution is neutralized with a neutralizing agent such as NaOH, NH4OH, etc., into a pH range stable against corrosion, ≥pH5 to give a combustion improver. The combustion improver is impregnated into or blended with a solid fuel such as coal or coke, etc.