A mass production system for synthesizing carbon nanotubes, comprising: a reaction chamber having at least one opening opened to external air, and at least one different-specific gravity gas occupying region filled with a different specific gravity gas having a different specific gravity from that of the external air to block the external air from being introduced into the reaction chamber through the opening; a carbon nanotube synthesizing unit positioned in the different-specific gravity gas occupying region to synthesize carbon nanotubes by the medium of a catalyst introduced thereto through the opening; a conveying unit to convey the catalyst to the carbon nanotube synthesizing unit through the opening; and a gas supply unit to supply the different specific gravity gas and a carbon source gas used for synthesizing the carbon nanotubes to the different-specific gravity gas occupying region and the carbon nanotube synthesizing unit, respectively, wherein the opening is opened downward to the external air, and wherein the different specific gravity gas that fills the different-specific gravity gas occupying region has a different specific gravity lower that that of the external air, and wherein the reaction chamber having the opening opened downward and located below the different-specific gravity gas occupying region filled with the different specific gravity gas having a different specific gravity from that of the external air so that the different specific gravity gas with the different specific gravity lower than that of the external air can be kept inside of the opening of the reaction chamber to keep the external air from being introduced into the reaction chamber through the opening. a reaction chamber having at least one opening opened to external air, and at least one different-specific gravity gas occupying region filled with a different specific gravity gas having a different specific gravity from that of the external air to block the external air from being introduced into the reaction chamber through the opening; a carbon nanotube synthesizing unit positioned in the different-specific gravity gas occupying region to synthesize carbon nanotubes by the medium of a catalyst introduced thereto through the opening; a conveying unit to convey the catalyst to the carbon nanotube synthesizing unit through the opening; and a gas supply unit to supply the different specific gravity gas and a carbon source gas used for synthesizing the carbon nanotubes to the different-specific gravity gas occupying region and the carbon nanotube synthesizing unit, respectively, wherein the opening is opened downward to the external air, and wherein the different specific gravity gas that fills the different-specific gravity gas occupying region has a different specific gravity lower that that of the external air, and wherein the reaction chamber having the opening opened downward and located below the different-specific gravity gas occupying region filled with the different specific gravity gas having a different specific gravity from that of the external air so that the different specific gravity gas with the different specific gravity lower than that of the external air can be kept inside of the opening of the reaction chamber to keep the external air from being introduced into the reaction chamber through the opening.