PURPOSE: To reduce the thickness of a muffle main body, to reduce the weight of a muffle, and to increase the length of a furnace by a method wherein the main body part, positioned in a furnace body, of the muffle for inserting a material to be treated vertically extending through the furnace body and suspended from the upper end thereof is formed of a material having different quality and high heat resistance.

CONSTITUTION: A heat treating device 1 is located to a building 3 on a foundation 2, and a heating furnace 5, a gradual cooling chamber 6, and a cooling chamber 7 are built in a state to be vertically interconnected. A muffle 13 is suspended in a state to vertically extend through a furnace body 10, and a suspension part 13 at the upper end of the muffle 13 is coupled to a muffle support truck 14. In which case, the muffle 13 comprises a main body part 13A positioned in the furnace body 10, a coupling part 13B, and a protruding and retracting part 13C protruded downward from the lower end of the furnace body. The main body part 13A is formed of a material, e.g. inconel 600, having high heat resistance. Meanwhile, the protruding and retracting part 13C is formed of a material, e.g. SUS 310S, having enough strength at a low temperature.