PROBLEM TO BE SOLVED: To provide a utilization method of a plastic waste in a firing furnace in which when the plastic waste is used as fuel in the firing furnace (rotary kiln or the like) to produce a burned product (calcium oxide, burnt dolomite or the like), attachment generation to a furnace wall part of an inside of the furnace by an impurity component (ash) included in the plastic waste can be prevented.SOLUTION: A utilization method of a plastic waste in a firing furnace is a method in which when a raw material in the firing furnace is heated by burning of fuel to produce a burned product, the plastic waste is used as a part of the fuel, and is characterized in that the plastic waste is added with an inorganic compound, and glass transformation temperature of a mixture of ash included in the plastic waste and the added inorganic compound is raised to at least 1250°C.