A method for manufacturing a thin-film semiconductor includes polycrystallization to focus visible light pulse laser into a line shape on a surface of an object to be irradiated, and repeat irradiation with displacing the visible light pulse laser such that a line-shaped irradiated region is overlapped with a region irradiated at a next timing in a width direction of the line-shaped irradiated region, to form a polycrystalline silicon film on the surface of the object. The step of polycrystallization applies ultraviolet light pulse laser onto a second irradiated region partially overlapping the first irradiated region while or before the visible light pulse laser is applied to the first irradiated region.