To suppress the generation of NOx and CO by a method wherein a water- cooled tube through which water fed to a boiler forcibly passes is arranged in such a state to approach the combustion part of a burner.

Water tubes through which upper and lower headers 1 and 2 are connected together are coupled together through a fin to form a pair of parallel water tube wall surfaces. A water tube for one thing approaching a burner 5 forms a water- cooled tube 4 and other tubes form a vaporizing tube. Upper and lower headers connected to the water-cooled tube 4 form a water-cooled tube upper header 8 and a water-cooled tube lower header 9, respectively. A steam separator 10 and the upper header 1 are connected together through a steam communication pipe 11 and the steam separator and the lower header 2 are connected together through a downcast pipe 12. A water feed line 3 is connected to the water-cooled tube lower header 9 and the water-cooled tube upper header 8 and the steam separator 10 are connected together through a feed water communication pipe 13. The temperature of combustion gas by a burner 5 is rapidly reduced to approximate 1300°C by the watercooled pipe 4. Since a time in which combustion gas resides at a high temperature is shortened, an amount of generating NOx and CO can be reduced.