An outer enclosed pressure container is formed of a simple steel material. An inner enclosed container is positioned within the pressure container such that there is an intermediate space therebetween. Catalyst layers are provided within the inner container. A gas containing a reducing component and water vapor are introduced into the inner container and are therein reacted by means of the catalyst to perform a desired reducing reaction. At least part of the water vapor is alone introduced into the intermediate space to thereby pressurize the intermediate space to substantially the same pressure as occurs within the inner container. The water vapor in the intermediate space operates to protect the wall of the pressure container from the temperatures occurring due to the catalytic reaction within the inner container. The water vapor is removed from the intermediate space and is at least partially added to the gas containing a reducing component before the introduction thereof into the inner container.