Disclosed is an antibacterial agent composition which is highly safe and has excellent antibacterial abilities, by using a silicon-containing compound that is obtained by a specific manufacturing method. The antibacterial agent composition has a more stable antibacterial component, and is capable of imparting antibacterial abilities to teeth, while being also capable of cleaning an article or the mouth. Also disclosed are: an antiviral agent composition which is highly safe and has excellent virus deactivation abilities; an antibacterializing method, a cleaning/mouth rinsing method, each using the antibacterial agent composition or the antiviral agent composition; and a method for fixing an antibacterial agent or an antiviral agent. The antibacterial agent composition may contain a silicon-containing compound which is represented by general formula (1) and obtained by reacting a specific triethoxysilyl compound in an ethanol solvent.