Abstract: A loading lock for a chemical vapor deposition apparatus comprises three chambers: an outer chamber for introduction and removal of substrates, a reaction chamber for forming thin films on the surfaces of the substrates and an intermediate chamber disposed between the outer chamber and the reaction chamber. The intermediate chamber keeps the reaction chamber in an air-tight state isolated from the atmosphere side at all times. As much of the mechanism for internal conveyance of the substrates as possible is located externally. This arrangement prevents oxidization of the substrate surfaces by air entering the apparatus during introduction and removal of the substrates. It also prevents generation inside the chambers of metallic contaminants, particles and the like which might adhere to the substrate surfaces.