PURPOSE: To reduce the heat absorption at a waterwall of a furnace and raise the temperature of exhaust gas to reduce unburnt content by a method wherein the upper part of the furnace is surrounded by a steam-cooled wall.

CONSTITUTION: The lower part of a furnace 8 is covered by a waterwall 9 and the upper part by a steam-cooled wall 27. Therefore, the feed water in a drum 6 flows down along a down-flow pipe 7, rises along the waterwall 9 extending from an inlet header 10 up to nearly the middle of the furnace 8, and flows out of an outlet header 11 to the drum 6 via connecting pipe 12. On the other hand, the steam in the drum 6 passes through a saturated steam pipe 13 and an inlet header 28, flows down along the steam-cooled wall 27, then to an outlet header 29, a steam pipe 30 and a mixing header 31 where it is mixed with steam coming from a cage wall 14, and finally into a primary heater 18 and a secondary heater 22. Thus, as the upper part of the furnace 8 is surrounded by the steam-cooled wall 27 through which stem flows, the heat absorption at the steam-cooled wall is reduced, resulting in a temperature rise of the exhaust gas. As a result, the unburnt content is reduced.