Disclosed is a method for forming a polycrystalline silicon film by crystallizing an amorphous silicon film. A mask has first to third shot regions having the same length. The first to third shot regions have transmission sections and non-transmission sections, which are alternately aligned. The transmission sections of the first shot region are positioned corresponding to the non-transmission sections of the second shot region, the non-transmission sections of the first shot region are positioned corresponding to the transmission sections of the second shot region, and the transmission sections of the third shot region are aligned corresponding to center portions of the transmission sections of the first and second shot regions. Primary to nth laser irradiation processes are performed with respect to the glass substrate, thereby crystallizing the amorphous silicon film into the polycrystalline silicon film.