PURPOSE: To stabilize a manufacturing process, improve safety and heighten an insulating property for raising reliability by sandwiching an insulating film on a grid between silicon nitride films to make three-layer constitution.

CONSTITUTION: The upper and lower sides of an insulating film 4 placed between a grid 3 on an insulating film 2 formed around a cathode 1 and an anode 5 are sandwiched between silicon nitride films 7, 8 to make three-layer constitution. With this constitution, contact between the grid 3 and the anode 5 due to over etching on a forming process of an active region 6 is prevented from occurring, and separation of a mask is suppressed against etching even for long time. Consequently, working in the forming process of the active region becomes easy while improving stabilization and safety, heightening an insulation property and raising reliability can be realized.