PURPOSE: To prevent a lump of incinerated ash contained in exhaust gas from dropping on an upper air hood at a time in a case an air duct has a horizontal portion.

CONSTITUTION: An atmosphere introduction pipe 19 through which the atmosphere 18 is introducible toward the upper surface of an upper air hood 4 is arranged at the lower part of the through-position of a horizontal 13 of an air duct 9 of an upper exhaust gas duct 1, an on-off valve 20 is located in the atmosphere introduction pipe 19, and the on-off valve 20 is opened at intervals of a specified time. Or, an atmosphere introduction pipe 21 through which the atmosphere 18 is introducible is arranged above the through-position of the horizontal 13 of the air duct 9 of the upper exhaust gas duct 1 in such a state to point to the upper surface of the horizontal 13 of the upper air duct 9. Further, an on-off valve 22 is located in the atmosphere introduction pipe 21 and the on-off valve 22 is opened at intervals of a specified time to scatter incinerated ash 16.