To reduce production cost by pulverizing a combustible solid synthetic resin to a specified size or below, throwing smaller pulverized grains as fuel into the front of a kiln for cement production with a calcining furnace and throwing larger pulverized grains as fuel into the calcining furnace.

When a solid synthetic resin such as plastics is pulverized, it is pulverized to about ≤10 mm from the viewpoint of pulverization cost. In the case of pulverized grains from a pulverizer with discharges slits of ≤2 mm opening, grains discharged from the slits and grains remaining in the pulverized are separately recovered. The former grains of ≤2 mm, preferably 0.5-2 mm maximum diameter are thrown as fuel into the front of a rotary kiln with a calcining furnace and the latter grains of >2 mm diameter are thrown as fuel into the calcining furnace as a preheater.