A laser-irradiation method which comprises a process for fabricating a semiconductor device, comprising: a first step of forming a thin film amorphous semiconductor on a substrate having an insulating surface; a second step of modifying the thin film amorphous semiconductor into a crystalline thin film semiconductor by irradiating a pulse-type linear light and/or by applying a heat treatment; a third step of implanting an impurity element which imparts a one conductive type to the crystalline thin film semiconductor; and a fourth step of activating the impurity element by irradiating a pulse-type linear light and/or by applying a heat treatment; wherein the peak value, the peak width at half height, and the threshold width of the laser energy in the second and the fourth steps above are each distributed within a range of approximately ±3% of the standard value. Also claimed is a laser irradiation device which realizes the method above.