To provide an IgA production promoter capable of being easily ingested safely and continually, for example orally and capable of preventing infection by inhibiting the binding of pathogenic microorganisms to alimentary canal mucosa and preventing an allergic reaction by blocking the passage of allergen materials such as pollen and house dust through the wall of the alimentary tract. The present invention consists of an IgA production promoter comprising hydroxypropyl starch as an effective ingredient. The IgA production promoter of the present invention can be safely and continually ingested because the effective ingredient thereof is hydroxypropyl starch used in food and the like, and can prevent infection by inhibiting the binding of pathogenic microorganisms to alimentary canal mucosa and prevent an allergic reaction by blocking the passage of allergen materials such as pollen and house dust through the wall of the alimentary tract.
