To provide a heat treatment furnace capable of preventing application of a gas of low temperature to a heated object and a housing, and reducing variation of a temperature in the furnace.

A gas supply pipe 22 and an U-shaped heater 26 are formed in a furnace main body 12 of the heat treatment furnace 10. A gas injection hole for injecting a gas is formed on the gas supply pipe 22, and the gas supply pipe 22 and the heater 26 are disposed so that an atmosphere gas injected from the gas injection hole is directly applied to the heater 26. The housing 32 provided with a ceramic laminated body is placed in the furnace, and the heat treatment is performed on the ceramic laminated body by heating by the heater 26 while supplying the atmosphere gas from the gas supply pipe 22 into the furnace. Here, the atmosphere gas is supplied into the furnace in a state of being heated by the heater 26.

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