PURPOSE: To eliminate the loosening of a bolt and a nut and to eliminate the leakage of oxygen from a flange part by fastening an expansion joint to the flange parts being the fastening parts of a muffle main body expanding and contracting by heat and a sub-muffle.

CONSTITUTION: An expansion joint 1 is molded by applying pressure to a square pipe. Corrugated parts 9 are provided to the central parts of the expansion joint 1 in order to buffer the expansion and contraction due to heat of a muffle main body 4 and a sub-muffle 3 and the degree of buffer to the expansion and contraction is determined by the wavy form of the corrugated parts 9. Since the upper and lower parts of baking are different in the quantity of expansion because of the difference in heat capacity, force is applied to the expansion joint 1. The expansion joint 1 absorbs the difference in the expansion and contraction to eliminate the loosening of the fastening parts of the muffle main body 4 and the sub-muffle 3 without applying stress to the bolts 2 and nuts 7 connecting both of them beyond tolerance.