PURPOSE: To stabilize combustion by providing a preheater in the upper part of a combustion furnace, leading a combustion gas to the preheater and equipping a filter having a shielding plate on the downstream side of a throttle plate provided on the downstream side of a secondary air hole.

CONSTITUTION: A preheater 106 is provided in the upper part of a combustion furnace 105 to lead a combustion gas to the preheater. Air for combustion is preheated to cool the combustion gas with a cooler 107. A filter 133 equipped with a throttle plate 131 is provided at a position opposite to the throttle plate on the downstream side of the throttle plate 131 on the downstream side of a secondary air hole 120 to reduce drafting effects and stabilize the mixed flow rate of the incomplete combustion gas with the secondary combustion air.