PURPOSE: To reduce heat loss by pressure supporting the cathode through the resiliency of spring coil against the cathode while contacting against a projection.

CONSTITUTION: The projection 12 from supporter 2 of electron emission surface 1 is contacted through ceramic ring 18 against the contacting body 5 formed integrally with tubular member 16 having spring coil supporter 19. One end of spring coil 15 is fixed through ceramic ring 14 to supporter 19 while the other end is fixed to the flange of supporter 2 and supported in compressed state. Consequently the thermal joint between the supporter 2 and ceramic ring 18 is in point contact state through a projection 12, while thermal joint between the supporter 2 and tubular member 16 is made through spring coil 15, the heat loss due to thermal conduction can be considerably reduced while relative position between cathode and other electrode can be maintained constant.