PURPOSE: To prevent a duct fire and improve a safety by a method wherein oily substances adhered to a grease filter or a smoke discharging duct are always discharged out of a smoke discharging duct through an oil discharging passage.

CONSTITUTION: In concurrent with an ignition of a burner 12, a discharging air blower 15 is operated so as to make an air flow flowing from a suction port 13 to a discharging port 16 and further to suck oily smoke or the like into a smoke discharging duct 17. In this case, the oily substances sucked into the smoke discharging duct 17 is adhered to the grease filter 23 arranged inside the suction port 13 and then removed. In turn, non-collected oily substance passed through the grease filter 23 is adhered to an inner surface or lower part of the smoke discharging duct 17, the substance is automatically dropped at the wall surface, collected at an oil discharging passage 18 arranged at a lower part and then the substance is discharged through the oil discharging hole 19 to an oil reservoir 20 arranged outside the smoke discharging duct 17. In addition, the oily substance adhered to the grease filter 23 is flowed down along a valley part of a net plate 21 due to the fact that a side having no frame 22 is inclined to be positioned at a lower part and then the oil is automatically dropped at the wall surface of the smoke discharging duct 17, the oil is collected at the oil discharging passage 18 arranged at a lower part and the oil is discharged out of the smoke discharging duct 17 through an oil discharging hole 19.