introducing steel material into a transportation room, wherein the transportation room is configured to transport the steel material between a carburization room, an oil quenching room, and a gas cooling room;

reducing the pressure in the transportation room to 0.1 kPa or lower;

transporting the steel material after the pressure of the transportation room has been reduced from the transportation room to the carburization room by way of opening a first partition door located between the transportation room and the carburization room;

elevating a pressure in the carburization room by introducing N₂ gas therein and elevating a temperature in the carburization room after the first partition door is closed;

reducing the pressure in the carburization room to 0.1 kPa or lower by discharging the N₂ gas;

supplying a predetermined amount of hydrocarbon gas and a predetermined amount of oxidative gas to the carburization room so that the internal pressure of the carburization room is 0.1 to 101 kPa, the hydrocarbon gas being at least one of gases selected from the group consisting of C₃H₈, C₃H₆, C₄H₁₀, C₂H₂, C₂H₄, C₂H₆ and CH₄;

controlling the amount of hydrocarbon gas and the amount of oxidative gas in the carburization room with the internal pressure of the carburization room kept at 0.1 to 101 kPa such that an atmosphere within the carburization room has a high carbon potential which is slightly below a carbon solid solubility limit;

stopping the supply of hydrocarbon gas and oxidative gas and reducing the pressure in the carburization room; and

after a predetermined amount of time, lowering the temperature in the carburization room to an oil quenching temperature and opening the first partition door;

transporting the steel material to the oil quenching room from the carburization room, via the transportation room by opening a second partition door located between the transportation room and the oil quenching room; and

performing an oil quenching treatment on the steel material in the oil quenching room.