This invention is intended to discover a fraction having strong anti-influenza virus activity via Mφ activation in the Grifola frondosa extract or an active substance distributed therein to develop a simple and effective production method and to use such fraction for food and beverage products, pharmaceutical products, feeds or feed additives, and the like. This is realized by treatment of maitake mushrooms with a molecular sieve apparatus, such as an ultrafiltration apparatus or gel filtration apparatus, to isolate glycoprotein-containing fractions or sugar-protein complex-containing fractions having molecular weights of 30,000 to 100,000, so as to minimize the influence on substances having strong anti-influenza virus activity and contaminants.
