The average film thickness of an amorphous silicon film formed on a substrate is measured. Then, the amorphous silicon film is irradiated with a laser beam to form a polysilicon film, and the grain size distribution of the polysilicon film is measured. An optimum value of energy density of laser beam irradiation is calculated on the basis of grain size values measured at two points A and B of the polysilicon film. Then, the average film thickness of an amorphous silicon film formed on a subsequent substrate is measured. A value of energy density of laser beam irradiation for the subsequent amorphous silicon film is calculated on the basis of the two average film thicknesses. Accordingly, a uniform polysilicon film of large grain sizes is formed on the whole surface of a large-size substrate to provide polysilicon TFTs in a large area.