A process for producing aqueous chlorous acid solution in which chlorous acid, which is safe for the human body, is easy to handle, and less generates chlorine dioxide, is yielded and used as a disinfectant for a pretreatment in food processing. To an aqueous sodium chlorate solution is added sulfuric acid or an aqueous solution thereof in such an amount and concentration that the pH of the aqueous solution can be kept at 2.3-3.4 to thereby react them and generate chloric acid. Subsequently, hydrogen peroxide is added to the chloric acid in an amount which is equal to or larger than the amount necessary for a reduction reaction to thereby yield chlorous acid. Any one of inorganic acids, inorganic acid salts, organic acids, and organic acid salts, or two or more thereof, or a combination or these is added to the aqueous solution containing chlorous acid yielded, whereby the chlorous and acid can be present for long and the pH of the aqueous solution is regulated to regulated to 3.2-7.0. Thus, high bactericidal power is imparted thereto.