The present invention relates to a pharmaceutical composition for inhibiting or preventing viral infection, which comprises an effective dose of poly-gamma-glutamic acid (γ-PGA), and more particularly, to a pharmaceutical composition, a functional food, and a feedstuff additive capable of inhibiting viral infection and preventing viral diseases, which comprise poly-gamma-glutamic acid having an infection-inhibiting effect against viruses, such as an influenza virus, inducing respiratory infection or systemic infection, as an effective ingredient.The composition containing poly-gamma-glutamic acid as an effective ingredient according to the present invention, is effective for use as an animal feedstuff additive or a pharmaceutical agent for preventing influenza virus infection and various viral diseases as well as a pharmaceutical composition and a functional food to promote human health.