Front substrate and back substrate of a PDP are respectively in a warped state such that a central portion of each substrate projects more frontwards than peripheral portions of the respective substrate, so that the front surface is convex. A stress remains in the substrates such that the two substrates are pressed to each other with an elastic deformation. In preparing the two substrates, the front panel and back panel are respectively warped towards each other so that the facing inner surfaces are convex in being sealed with each other. A height difference ratio of the central portion from a central part of a short side of the back substrate is preferably less than 0.16%. A height difference ratio of the central portion from a central part of a short side of the front substrate is preferably less than 0.06%. Difference of the height difference ratios of the back substrate and the front substrate is preferably 0 to 0.1 point.