Provided is a highly safe antiviral agent which exhibits an effect through preventive ingestion and which exhibits an effect on any kind of virus through intake. A neutral polysachharide to be produced by Lactobacillus delbrueckii ssp. bulgaricus OLL1073R-1 enhances innate immunity by enhancing an increase in activity of an NK cell in response to IFN-a and provides a preventive effect which is free of viral specificity unlike a vaccine and which can be exhibited before viral infection.
