A gate insulating film (13) is formed on a substrate (1) so as to cover a gate electrode (11), and an amorphous silicon film (semiconductor thin film) (15) is further formed. A light absorption layer (19) is formed thereon through a buffer layer (17). Energy lines Lh are applied to the light absorption layer (19) from a continuous-wave laser such as a semiconductor laser. This oxidizes only a surface side of the light absorption layer Lh and produces a beautiful crystalline silicon film (15a) obtained by crystallizing the amorphous silicon film (15) using heat generated by thermal conversion of the energy lines Lh at the light absorption layer (19) and heat of the oxidation reaction. This provides a method for crystallizing a thin film with good controllability at low costs achieved with simpler process.