a phototreating apparatus has a vacuum container for storing a solid material , a gas inlet for introducing a photoreactive gas into the container , and a light source for radiating into the container , light having a specific wavelength such that it causes a photoreaction of the photoreactive gas . the apparatus phototreats the solid material in the container by utilizing the photoreaction of the photoreactive gas . at least an inner surface of the vacuum container consists of a material which absorbs the light having the specific wavelength .