To provide a method for keeping airtightness without causing the position slippage of a face plate and a rear plate in a method for sealing upper and lower plates with frit glass in the process for assembling a plane image forming device.

A seating having middle height of a crushing margin is joined to the lower plate, a flat spring having a coefficient of thermal expansion similar to that of glass is joined to the upper plate at several points, and after alignment, the seating and the flat spring are joined. In sealing, a load is applied through a ball between a pressing plane and the upper plate.