PURPOSE: To prevent heat dissipation of a heater and shorten preheating time by forming a part or the whole of a cathode supporting body of an M type tube out of a ceramic and potting the heater with alumina.

CONSTITUTION: Heat dissipation caused by conduction to a cathode supporting body is restrained by a ceramic cathode supporting body 3 and the heat dissipation caused by heat radiation to a heater inner circumferential part is restrained by a potting part 4 with alumina. With this constitution, power supplied to a heater 2 is reduced and the heater 2 is firmly fixed by potting with alumina, thereby breaking of the heater or the like due to vibration and shock of a tube can also be prevented.