A method of manufacturing a semiconductor device includes modifying a first laser beam from a first laser to form a first linear-shaped laser beam and modifying a second laser beam from a second laser to form a second linear-shaped laser beam. The method further includes overlaying the first linear-shaped laser beam and the second linear-shaped laser beam to form an overlayed linear-shaped laser beam, wherein the overlayed linear-shaped laser beam has a width and a length where the length is ten times or more as large as the width. The method also includes scanning a semiconductor film formed over a substrate with the overlayed linear-shaped laser beam to increase crystallinity of the semiconductor film, and patterning the semiconductor film to form a semiconductor layer which includes a channel formation region of a transistor.