With a cover member being located directly upward of an opening of a casing, a substrate in a horizontal attitude is carried into a gap between the cover member and the opening. The carried-in substrate is moved to the inside of the casing through the opening. The cover member is lowered, so that the opening is closed. An inert gas is supplied to the inside of the casing. One surface of the substrate is irradiated with vacuum ultraviolet rays by a light emitter while the substrate is moved in a horizontal direction in the casing. The cover member is lifted, so that the opening is opened. The substrate is moved to a position between the cover member and the opening from a position inside of the casing through the opening. Thereafter, the substrate in the horizontal attitude is taken out in the horizontal direction.