PURPOSE: To activate the electrode rapidly by contacting the outer periphery of an impregnated cathode and a metallic sleeve, and making the temperature rise of a cathode easier by a high-frequency heating through the sleeve.

CONSTITUTION: A tubular matallic sleeve 3 connected to the periphery of an impregnated cathode 2 made by impregnating an electron radiating substance to a high melting point heat resistant metal power sintered body is provided, and a linear heating body 5 is buried in the cathode 2 to form an electrode 1 for a fluorescent tube. When a high-frequency heating is applied when the electrode 1 is activated, the cathode 2 is heated rapidly by heating the sleeve 3, and an oxide attached on the surface can be removed rapidly. And in the radiation after the electrode 1 is assembled, the sleeve 3 discharges the emission for the period until the cathode discharges the emission, because the work function of the sleeve 3 is small. Furthermore, by the heating by the electron flow-in to the heating body 5, the cathode 2 is heated to accelerate the temperature rise, so as to make a high temperature in a small current, and thereby, the emission is discharged easily.