A heat treating unit for semiconductor processing is adapted to conduct normal pressure high temperature processing and low pressure thermal processing using corrosive gases. The unit includes an inner tube for receiving a boat which carries objects to be processed, an outer tube concentrically disposed outside the inner tube, a cylindrical manifold which has a gas feed port and an exhaust port, and a cap which tightly closes an opening of the manifold. The inner tube, outer tube and manifold are formed of quartz which is heat resistant and corrosion resistant, and these three components are integrally joined together by melting. The interior surface of the cap is provided with a protecting layer which is heat resistant and corrosion resistant. A connection between the cap and the manifold includes a high temperature heat resistant seal in which O-rings are cooled by a cooling system.