A slightly-acidic aqueous hypochlorous acid solution containing ultrafine bubbles, which has a high antiseptic effect, can be used safely even in foods, has excellent thermal stability, and can be stored for a long period. A hypochlorous-acid-containing aqueous solution which is adjusted to a slightly acidic range by dissolving carbon dioxide or both carbon dioxide and a nitrogen gas therein, and which contains ultrafine bubbles having an average particle diameter of 0.05 to 2 μm. In the ultrafine bubbles, a gas such as carbon dioxide and a nitrogen gas is contained. Carbon dioxide or both carbon dioxide and a nitrogen gas is mixed with and dissolved in electrolytic hypochlorite water or an aqueous sodium hypochlorite solution, wherein the effective chlorine concentration of the electrolytic hypochlorite water or the aqueous sodium hypochlorite solution is adjusted to a desired value. In this manner, the pH value of the water or the aqueous solution is adjusted to 5 to 6. The aqueous solution which contains hypochlorous acid and into which carbon dioxide or both of carbon dioxide and a nitrogen gas have been dissolved is allowed to pass through a microbubble generation nozzle to generate ultrafine bubbles containing carbon dioxide or both carbon dioxide and the nitrogen gas. In this manner, a slightly-acidic aqueous hypochlorous acid solution containing ultrafine bubbles which contains carbon dioxide or both carbon dioxide and a nitrogen gas can be produced.
