To prevent an unfavorable combustion of a grille burner from occurring by providing a draft generating burner, which accelerates the discharging of a combustion gas in a grille chamber by a draft force, in the initial stage of an ignition.

The combustion gas of a draft generating burner 61 raises the temperature of air in an exhaust chamber 71, and generates a large temperature difference between the upstream and the downstream of an exhaust port 70, and generates an upward draft force in the exhaust chamber 71. By the generation of the draft force, the combustion gas from a grille burner 63 is smoothly discharged to the outside of a grille chamber 60 through the exhaust chamber 71. At the same time, the generation of the draft force sucks in the outside air from an air introduction port 65 on the bottom surface of the grille chamber 60. The outside air is sucked into a cooling path 64, and is made to flow along the lower surface of a receiving tray 62 and cools the receiving tray 62. By this constitution, an unfavorable combustion of the grille burner 63 can be prevented from occurring.