The purpose of this study was to compare the preventive effects of the slightly acidic hypochlorous acid (HOCl) mouthwash and chlorhexidine (CHX) mouthwash as a chemical plaque control agent for oral diseases. This study was conducted for 15 patients who had no systemic disease among the patients who visited M Dental Clinic in Busan, South Korea, excluding the patients with incomplete data. The patients were divided into three groups: five patients in the 0.005% CHX (Bukwang Pharm Co., Ltd.) gargling group, five in the slightly acidic HOCl gargling group, and five in the saline gargling group. The patients were instructed to gargle with 15 ml of each mouthwash for 1 minute, and to then spit saliva for 1 min to remove the remaining gargling solution. Then the halitosis, O’Leary index, dental caries activity, bacterial species, and motility were measured. As a result, these decreased in the CHX gargling group compared to the saline gargling group, but the difference was not statistically significant (P>0.005), while there were significant decreases in the HOCl gargling group. The O’Leary index, Snyder test, bacterial motility, and Filamentous more distinctly and effectively decreased in the HOCl gargling group than in the CHX gargling group, showing statistically significant differences (p<0.05). Based on this study, CHX, which is currently widely used in dentistry, can be said to be inconvenient to use due to its strong fragrance and taste, and there have been many reports on the side effects on the oral cavity of its long-term use. Therefore, if HOCl, which is harmless to the human body and has an extensive sterilizing power, is used instead of CHX, safe and effective oral care will be achieved.