In producing a thin film transistor (TFT), an silicon oxide film is formed as an under film on a glass substrate, and then an amorphous silicon film is formed therein. A metal element which promotes crystallization of silicon is disposed in contact with a surface of the amorphous silicon film. A thermal processing for the amorphous silicon film is performed at a crystallization temperature of the amorphous silicon film or higher. At the thermal processing, a glass substrate is placed on an object having constant flatness. Cooling is performed to obtain a crystalline silicon film wherein the substrate is not distorted and deformed.