[PROBLEMS] To effectively utilize coal ash while lowering the concentration of mercury in a gas discharged from a cement kiln. [MEANS FOR SOLVING PROBLEMS] Coal ash from a thermal power plant or the like is received by a system for producing cement. The received coal ash is divided into ash and unburnt carbon. The ash thus divided is utilized as a material for cement in the system for producing cement, while the unburnt carbon thus divided is utilized in the system for producing cement depending on the concentration of mercury in a gas discharged from a cement kiln of the system for producing cement. In the case where gags discharged from the cement kiln of the system for producing cement contains mercury at a high concentration, it is possible that the unburnt carbon in the coal ash, which contains mercury localized therein, is not fed into the system for producing cement but processed outside the system for producing cement or the unburnt carbon is fed in a controlled amount into the system for producing cement. Mercury is removed form the thus separated unburnt carbon and the unburnt carbon, from which mercury has been removed, can be used as a fuel in the system for producing cement.