To provide a method for preventing closure of a furnace bottom of a cyclone type incinerator by mixing low calorific value sludge, when the unit calorific value of sludge for incineration and heat quantity of incineration reach a prescribed value, for reducing the unit calorific value to restrict generation of molten solid (so-called clinker) as a cause for closure of the furnace bottom of the cyclone type incinerator.

In this method for preventing closure of a furnace bottom in a cyclone type incinerator for incinerating sludge for incineration discharged from paper mills, when the unit calorific value of the sludge for incineration exceeds 11600 kJ/kg, with the unit calorific value exceeding 26700 MJ/h, low calorific value sludge of a unit calorific value of 3400-6700 kJ/kg is mixed to reduce the unit calorific value of the sludge for incineration. Closure of the furnace bottom in the cyclone type incinerator can thus be prevented without reducing incineration quantity.