A method is obtained of manufacturing a semiconductor device including a semiconductor layer with high field-effect mobility. According to the semiconductor device manufacturing method, a semiconductor layer is formed on a substrate and then the semiconductor layer is irradiated with high energy beam. Then, a heat treatment is provided under a temperature condition capable of reducing the surface roughness of the semiconductor layer. The radiation of high energy beam toward the semiconductor layer improves the crystalinity of the semiconductor layer and the subsequent heat treatment reduces the surface roughness of the semiconductor layer to enhance the field-effect mobility of the semiconductor layer.