A thin film semiconductor device having improved operating characteristics and reliability of a thin film transistor formed on a glass substrate. The thin film semiconductor device has a thin film transistor 3 formed on a glass substrate 1 containing alkali metal. The surface of the glass substrate 1 is covered by a buffer layer 2. The thin film transistor 3 formed on this buffer layer 2 has a polycrystalline semiconductor thin film 4 as an active layer. The buffer layer 2 includes at least a silicon nitride film and protects the thin film transistor 3 from contamination by alkali metals such as Na and has a thickness such that it can shield the thin film transistor 3 from an electric field created by localized alkali metal ions (Na+).