An antimicrobial agent with a high degree of safety is provided, which is derived from a natural product and can exhibit growth-inhibitory activity against acid-resistant and heat-resistant bacteria such as Alicyclobacillus acidoterrestris, which is resistant against pasteurization and causes spoilage of fruit juice. The antimicrobial agent against acid-resistant and heat-resistant bacteria contains as an effective ingredient alpha-type thionin and/or beta-type thionin. A preservative for fruit juice is also provided, which contains as an effective ingredient the alpha-type thionin and/or beta-type thionin.