An interlayer insulator with a low taper angle is formed as a laminated film in which a silicon nitride film is formed on a silicon oxide film formed on the glass substrate side (field insulator side). Thus, an upper electrode of a cathode formed on the interlayer insulator is prevented from being broken, while a crossing portion between the upper electrode and a base electrode of the cathode is made low in capacitance. At the same time, sodium separated from glass of the substrate is blocked. Disconnection of the upper electrode is prevented due to the low taper angle of the interlayer insulator. Low capacitance is attained by increasing the film thickness of the interlayer insulator. The cathode is prevented from being contaminated with sodium separated from glass of the substrate.