To improve the cooling efficiency of a generated gas by injecting a cooling water in the tangential direction to horizontal cross-section of a downcomer so that the wetted-wall flow of the cooling water on the inside surface of a downcomer becomes a swirling flow in the same direction as or the reverse direction to the swirling flow of the generated gas at the time of cooling the generated gas by supplying the cooling water.

In the high temp. rotary furnace constituted of a combustion section 1 and a gas quenching section 2, a gasified gas (a), oxygen (b) and steam (c) are blown in the tangential direction of swirling flow to form the swirling flow. The generated gas from the combustion section 1 is passed through a throat part 3 while keeping the swirling flow and enters into a gas quenching section 2. Though the inside of the downcomer 4 of the gas quenching section 2 is exposed to high temp. atmosphere by passing the high temp. generated gas and a molten slag, the cooling water (e) is supplied to the inside surface of the downcomer 4 in the tangential direction to horizontal cross-section or the downcomer 4 in a cooling water injection part 9 to form the wetted-wall swirling flow, thus the damage of the material is evaded and the sticking solidified slag is allowed to flow down into a water tank 5.