In the beam expander 10 of the present embodiment, the valves 31A-31D are opened while oxygen in the sealed spacers 30A-30D is absorbed, and gas such as nitrogen N2 or helium He is filled into the sealed spaces 30A-30D to the volume corresponding to the reduced volume of oxygen so that the change of refractive index due to the pressure difference between the external space and the sealed spaces 30