To prevent efficiently the distortion and deformation of a firing space due to the thermal expansion of a muffle.

A muffle 3 is supported at the upper part so that the muffle's own weight may not be applied thereto substantially, while the muffle 3 is sealed by bringing a hearth 5 into contact with the open lower end part of the muffle 3, and a stress caused by the thermal expansion-contraction of the muffle 3 is absorbed by providing a stress absorbing mechanism part 6 at a position being lower than the supporting part. The structure of the stress absorbing mechanism part 6 is made a bellows structure. While the muffle is supported at the upper part so that the its own weight may not be applied thereto, a seal member constituted of an elastic material which can absorb the thermal expansion-contraction of the muffle is disposed at the part of the contact of the hearth with the muffle, and the muffle is sealed by bringing the seal member into contact with the lower end part of the muffle.