PURPOSE: To enable direct feed of long and large diameter items to be incinerated and tight-sealed combustion by a method wherein an annular groove is provided around the brim of an upper opening and filled with water, and a cover which covers the upper opening consists of a hollow cover body which includes evaporation tubes inside and an annular seal plate extending from the cover body into cooling water in the annular groove.

CONSTITUTION: When a cover 122 is removed, an upper opening 114 has a large rectangular shape, so that long and large diameter items to be incinerated, such as demolished building debris, extra-large tires and others can be directly fed into a furnace 110 without pretreatment for size reduction and piled up on a bottom cover 129 in turn until the furnace filled with the items to be incinerated. The charge amount is limited to such an extent that incinerated residue can be accommodated within a small diameter part 112 without covering a sloped part 113. When the cover 122 is placed so as to cover the upper opening 144, an annular seal plate 125 is satisfactorily inserted into a groove 118. In this state, a jacket 116, a compartment 123, the bottom cover 129 and the groove 133 are filled full with water and the upper opening 114 and a bottom opening 115 are completely water-sealed.