PURPOSE: To prevent the temperature rise of a metal cap by performing high frequency heating to the material to be heat-processed in the metal cap under the condition that one end of a glass tube sealed with the metal cap and the low melting point glass is soaked in the liquid.

CONSTITUTION: In the manufacture of a cold cathode lamp 9, firstly, a glass tube 2 is sealed with a metal cap 11, and thereafter, one end of the cap 11 with an electrode 10 having the getter material 16 of the glass tube 2 and mercury alloy 17 is soaked in the water 13 of a water tank 14, and is maintained at the fixed position near a high frequency electric field generating means 15 in the water. The means 15 is operated under this condition to perform the high frequency heating to the electrode 10 in the glass tube 2 and activate the material 16 and vaporize mercury from the alloy 17. At this stage, the high frequency heating is also performed to the cap 11, but, since the surface of the cap 11 is soaked in the water 13 and cooled, the temperature of the cap 11 does not rise to more than 100°C. consequently, the danger of the low melting point glass 12 being melted is eliminated.