Abstract: A method for preparing a vaccine for dental caries comprises the step of culturing a variant which is obtained by integrating a protein antigen (PAc)-expressing gene into the chromosomal gene of a Streptococcus mutans GS-5 strain to obtain the protein antigen, the protein antigen being produced on the surface of cells of oral Streptococcus mutans or it being extracellularly produced by the microorganism and having a molecular weight ranging from about 170,000 to 220,000. Streptococcus mutans GS-5 (K-3), in which a protein antigen-expressing gene is integrated into the chromosomal gene thereof, has an ability of producing the protein antigen on the surface of the cells or extracellularly. A preventive vaccine composition for dental caries for nasal drops comprises the protein antigen thus produced by the strain: Streptcoccus mutans, the vaccine being intranasally administered. The method makes it possible to enhance the yield of PAc and to simplify processes for purifying PAc.