To provide a heat treat furnace capable of easily and surely preventing carbon from depositing inside a gas inlet pipe or a sample gas outlet pipe, and to provide a method of operating the same.

The method of operating a heat treat furnace 1, having a furnace casing 11 for receiving in a furnace 110 a material 6 to be processed and heat-treating the same and a gas inlet pipe 12 for introducing a hydrocarbon-based addition gas into the furnace during the heat treatment comprises the steps of stopping the introduction of the addition gas into the furnace through the gas inlet pipe 12, and thereafter introducing a gas, having a pressure higher than that of the furnace into the furnace casing 11 through the gas inlet pipe 12, so as to remove the addition gas from the gas inlet pipe 12.