The present invention addresses the problem of providing an antibody titer-increasing agent, such as an adjuvant, capable of sustaining an effect achieved by, for example, vaccination, which is an antibody titer-increasing agent such as an adjuvant that is safe, convenient and economical and can be easily taken irrespective of age, and a method for manufacturing the same. A method for increasing an antibody titer and a method for enhancing the effect of a vaccine, each method comprising using Lactobacillus delbrueckii subspecies bulgaricus OLL1073R-1 or a culture thereof and an antibody titer-increasing agent such as an adjuvant that comprises Lactobacillus delbrueckii subspecies bulgaricus OLL1073R-1 or a culture thereof.
