PURPOSE: To perfectly execute bright heat treatment to an active metal by detecting various gases generated in the process of heat treatment by a mass spectrometer and controlling the substantial temp. of the active metal based on the above detected value.

CONSTITUTION: The inside of a vacuum chamber 1 is provided with a heat insulating material 2, a heater 3 and a mass spectrometer 6. After the start of the temp. rising, hydrogen, oxygen, nitrogen and a hydrogen gas generated in a furnace are monitored by a mass spectrometer 6. The substantial temp. of an active metal 7 is regularly measured by a thermometer 8. The substantial temp. of a product at the time of detecting the generation of outgas by the mass spectrometer 6 is checked. In the case the substantial temp. has not reached a certain one at which outgas is fully absorbed, the heat treatment is continued as it is. In the case the substantial temp. has reached a certain one or above, the heater 3 is turned off, and the substantial temp. is reduced to a one at which outgas is not absorbed. In the meantime, a vacuum pump 5 is operated to exhaust the outgas in the furnace. When the outgas disappears, the heat treatment is restarted.