PURPOSE: To improve working stability as well as an yield while preventing tube damage by giving speific value to the frequency of a high frequency heating device which heats a getter device.

CONSTITUTION: Getter flushing is performed by making a high frequency current to flow through a coil 7. Thereby, the temperature of a getter device 4 rises up to about 900W1,000°C 5W10sec after starting the high frequency heating and a getter agent 4b is evaporated while generating creepage discharge between a getter film 10 and a spring 5. Therefore, when the frequency of the high frequency heating device is lowered down to 200W350kHz, the creepage discharge starting voltage is raised thus preventing creepage discharge. Accordingly, tube damage is excluded while improving working stability as well as an yield.