PURPOSE: To enrich gas calorie of circulating gas at low cost and to increase the amount of steam generated of a boiler, without lowering strength of coke, by feeding high polymer waste to a moving layer of red-hot coke in a cooling column and thermally decomposing.

CONSTITUTION: A moving layer of red-hot coke 1 is formed from the upper part to the lower part in a cooling column 2. High polymer waste 3 is added to the moving layer and the prepared decomposition gas is burnt in an exhaust system of the cooling column 2. A circulating gas for cooling is blown upon the moving layer of red-hot coke in counter current from the bottom of the cooling column and heat is recovered from the circulating gas which is subjected to heat exchange with the red-hot coke and discharged from the cooling column 2.