PURPOSE: To make it possible to attain perfect combustion for an article to be burned by installing a bending plane which forms a vent passage as opposed to an inner wall surface to a cylinder which is in contact with the inner wall surface of a furnace body and boring an air hole on the bending plane.

CONSTITUTION: A bending plane 6a is formed by bending a metal plate which constitutes an inner furnace at proper locations. It is provided to each corner portion of a furnace main body 1 as opposed, which makes it possible to form a triangle column-shaped air vent passage T at the corner portions and the bending planes 6a. A proper number of air holes K are bored on the bending planes 6a so as to introduce the air in the air vent passage T into the furnace. This construction makes it possible to supply a proper amount of air into the furnace in a stabilized manner. In addition, the double structure of the inner furnace and the furnace main body enhances the strength of the furnace and attains perfect combustion for disposed daily products.