PURPOSE: To prevent a tube from breakage of expansion caused by pressure difference by adjusting the pressure in the tube in radiant tube heater composing the heater body and the tube so as to equalize to the pressure in the heating chamber.

CONSTITUTION: The radiant tube heater 11 is constituted of the heater body 12 and the tube 13 covering this and in the heating chamber 3 of a vacuum treating furnace 1, after vacuum heating together with degreasing the material M to be heated, it is quenched. Then, the pressure in the tube 13 and the pressure in the heating chamber 3 are detected by the pressure detecting instrument 28, and the pressure in the tube 13 is the almost same pressure as the pressure in the heating chamber 3 by working of a pressure adjusting device 24. By this method, the breakage of the tube 3 by expansion caused by pressure difference is prevented and oil attached on the surface of the material M to be heated is vaporized, to execute the degrease and heating at the same time.