A window material for an optical element, including: a synthetic quartz glass substrate having a flat plate shape and having main surfaces through which light is transmitted, at least one of the main surfaces being a rough surface; and an antireflection film formed on the at least one main surface of the synthetic quartz glass substrate, the main surface being the rough surface. The window material for an optical element of the present invention is easy in shape processing, undergoes little temporal change in a wide wavelength region and is stable over a long period of time, and has high total light transmittance of distributed light.