To obtain high transparency by applying sol solution in which fine particles of Mg hydroxide combined with polyhydric alcohol or its derivative is dispersed in organic solvent containing organic compounds having hydroxyl groups and drying and calcining the coating film.

Polyhydric alcohol such as ethylene glycol or its derivative in an amount of 50-950 pts.wt. (hereafter abbreviated as pts.) and a prescribed quantity of water and NH3 water are added to 100 pts. of a Mg compound such as MgCl2, and mixed to form Mg composite oxide fine particles. To 100-50 pts. of organic solvent containing hydroxyl groups such as ethanol and glycerin, 100 pts. of the Mg composite oxide fine particles are added, and the particles are dispersed by ultrasonic dispersion or the like to prepare uniform sol solution. Next, when a AC type plasma display is formed, the sol solution is applied on a dielectric layer 5 with which an X electrode 4a and a Y electrode 4b are covered on a front substrate 1 opposite to a back substrate 2 arranged both across a discharge space 3, and is dried at about 200-300°C and then is calcined at 350-550°C for 1-5hr to form a MgO film protective layer 6 of ≤2μm thickness.