A laser beam treatment device capable of solving problem in a conventional technology that any uniform laser anneal cannot be realized since use of a galvano mirror changes the angle of incidence of the laser beam to the substrate and the reflected light from a back side of a transmissive substrate interferes with the reflected light from a surface of a semiconductor film or an interface between the semiconductor film and the substrate. Laser anneal is performed by using the laser beam treatment device comprising a laser, an optical system for shaping the laser beam oscillated from the laser, and a substrate holds to hold a work formed on the transmissive substrate, in which the substrate holder holds a liquid, and the liquid is brought into contact with the surface.