PURPOSE:To provide an antibody capable of inhibiting the formation of plaque to prevent dental caries, by preparing from eggs laid by chickens immunized with Streptococcus.mutans. CONSTITUTION:The objective antibody is an immunoglobulin prepared from eggs laid by chickens immunized with Streptococcus.mutans of blood serum types of c, d, e, f and g as dental caries-inducing pathogenic bacteria and has an immunologic activity against the dental caries-inducing pathogenic bacteria. IgG among the immunoglobulin is obtained from the yolk of the aforementioned eggs and IgM and IgA are also obtained from the egg albumen thereof. The immunoglobulin causes the aggregation of the Streptococcus.mutans, and it is preferable that the egg yolk is collected and purified to provide the objective antibody, because the IgG is particularly excellent in an effect inhibiting the adhesion of the Streptococcus.mutans to smooth surfaces. The antibody, e.g., the antibody having an aggregated antibody value of >=10 , can be compounded with the dental caries-preventing agent in an amount of approximately 0.0001-10wt.%.