A heat-treating apparatus capable of realizing a highly precise processing maintaining a high degree of safety, and a method of producing substrates are provided. The heat-treating apparatus comprises a reaction tube for treating substrates; a manifold for supporting the reaction tube; and a heater provided surrounding the reaction tube to heat the interior of reaction tube; wherein the reaction tube and the manifold are in contact with each other as their continuous flat surfaces come in contact with each other; a cover member is provided to cover the contact portion between the reaction tube and the manifold from the outer side; and the cover member is provided with at least either a gas feed port or an exhaust port communicated with a space formed among the cover member, the reaction tube and the manifold.