PURPOSE: To maintain a pressure difference between a furnace and the duct for combustion air substantially at a constant value irrespective of boiler load being imposed and to obtain a stabilized combustion state, by allowing an air sleeve to vary the end diameter thereof facing a furnace in response to a combustion air flow amount in a boiler.

CONSTITUTION: While boiler is operated under a reduced load, a manipulation rod 29 is pulled in the direction of arrow mark B, whereby causing narrow plate-shaped means 22, 24 to rotate in the direction of arrow mark D with a hinge 25 as fulcrum 25 and a variable air sleeve 21 to reduce the end diameter thereof facing the furnace. Therefore, the passage of combustion air flowing through a throat section 7 is blocked and an air pressure inside an air duct 12 is raised, whereby properly adjusting the pressure difference between a furnace 2 and the air duct 12. As it is unnecessary to operate the boiler with an excessive air amount, an oxygen concentration in the exhaust can be reduced and such an exhaust can be supplied as an inert gas into a cargo tank aboard a ship. Moreover, an economical advantage is also secured by eliminating the need of operating a boiler under an excessive air amount.