It is an object of the present invention to provide a high reliable EL display device and a manufacturing method thereof by shielding intruding moisture or oxygen which is a factor of deteriorating the property of an EL element without enlarging the EL display device.In the invention, application is used as a method for forming a high thermostability planarizing film 16, typically, an interlayer insulating film (a film which serves as a base film of a light emitting element later) of a TFT in which a skeletal structure is configured by the combination of silicon (Si) and oxygen (O). After the formation, an edge portion or an opening portion is formed to have a tapered shape. Afterwards, distortion is given by adding an inert element with a comparatively large atomic radius to modify or highly densify a surface (including a side surface) for preventing the intrusion of moisture or oxygen.