Abstract: A photochemical vapor phase reaction apparatus and a method of causing a photochemical vapor phase reaction are described. The apparatus comprises a vacuum chamber, a substrate holder provided in the vacuum chamber for holding a substrate to be treated by a vapor phase reaction, a gas feeding system for supplying a reactive gas to the reaction space, a light source housed in a light source room for emitting light rays through a light window, an optical system for condensing and projecting the light rays emitted from the light source onto the substrate on the holder. By this configuration, the intensity of light is relatively low at the light window and relatively high at the surface of a substrate to be treated.