PURPOSE: To make the air suction hole of a burner in a combustion part difficult to be clogged with dusts despite its use for a long period of time and lengthen a periodic inspection interval by a method wherein an inverted U-shaped air suction passage is formed by a filter and a cover and the projection area of this air suction passage is made larger than the area of an air suction opening.

CONSTITUTION: The combustion air is sucked in by an air blower 4 through a filter 6 small in air resistance and then through an air suction opening 5. Since the filter 6 is increased in air resistance due to the clogging thereof caused by accumulation of dusts thereon taking place in use for a long period of time, the combustion air is drawn in from the air suction opening 5 through an inverted U-shaped air suction passage 8. Since the air passage area of the air suction passage 8 is made larger than the area of the air suction opening 5, there is no air resistance in the air suction passage 8 creating the same conditions as in the case without air suction filter device. By the provision of the air suction filter device in this way, a periodic inspection interval can be lengthened by the time during which the dusts are accumulated on the filter 6 to cause its clogging.