A blow molding machine is subjected to a sterilizing treatment in a simple manner. A liquid sterilizer is filled into preforms of at least the same number as the number of spindles. In a heating unit that heats the preforms to a molding temperature, some of the liquid sterilizer is caused to diffuse, and blow air is blasted into preforms in which the liquid sterilizer remains to cause the liquid sterilizer to diffuse in a molding unit, and molded bottles are discharged.
