PURPOSE: To perform rapid and reliable detection of breakage of a muffle by detecting specified gas for detection present between a muffle and a furnace shell.

CONSTITUTION: When breakage of a muffle 22 occurs, gas in the muffle 22 leaks to a gap between the muffle 22 and a furnace shell 23. Gas for detection is mixed in the gas. The gas for detection is not contained in the air or an extremely slight amount of the gas is contained in the air. Besides, even when the gas for detection leaks to the outside of the muffle 22, it is not chemically changed through compounding, for example, and it is not burnt by a burner 24 for heating the muffle 22. The gas for detection is detected by a gas detecting means 30. This device enables rapid and reliable detection of the occurrence of breakage to the muffle 22.