To crystallize uniformly a semiconductor thin film constituting the active layer of a thin film transistor, by projecting a laser beam on it.

A thin film transistor has the laminated structure including a polycrystal semiconductor thin film 5, a gate oxide film 3 formed contacting it with the one-surface side of the film 5, and a gate electrode 1 superimposed on the polycrystal semiconductor thin film 5 via the gate oxide film 3. The thin film transistor is formed on an insulation substrate 0 at a process temperature not higher than 600°C. Projecting on an amorphous semiconductor thin film 4 formed previously a laser beam 50 excited impulsively and shaped sectionally in the form of a rectangle, the film 4 is converted into the polycrystal semiconductor thin film 5. In this case, the laser beam 50 with its pulse duration exceeding about 25 ns is projected on the same place at a lower frequency than five shots.