To provide a heat treatment furnace which ensures safety by a simple constitution when pressure inside a quenching chamber becomes negative and enables reduction of a modifying gas.

The heat treatment furnace 10 has a heating chamber 4, an oil tank 5, a quenching chamber 2, an exhaust pipe 9, and a nitrogen gas feed line 20. A workpiece 8 is held in the heating chamber 4 and heat-treated in an atmosphere of a predetermined modifying gas. A quenching oil is stored in the oil tank 5. The quenching chamber 2 is disposed contiguously to the heating chamber 4. The workpiece 8 after the heat treatment is conveyed into the quenching chamber 2 and the work piece 8 is quenched by being immersed in the oil in the oil tank 5. The exhaust pipe 9 is connected to the quenching chamber 2 and discharges the modifying gas used in the heat treatment. The nitrogen gas feed line 20 is connected to the exhaust pipe 9 and feeds a nitrogen gas into the exhaust pipe 9.