Influenza virus (Flu) A type that breaks out every year has multiple subtypes and mutates the antigenicity every year. For this reason, it is hard for us to prevent the infection by vaccine. Furthermore, anti-Flu drugs have many troubles. On the other hand, it has been reported that flavonoids of black tea and green tea have an anti-Flu function. Therefore, we noticed cassis containing many flavonoids such as anthocyanin, and investigated an anti-Flu function using in vitro and in vivo tests. Cassis extract (CE) inhibited hemagglutination caused by Flu, and reduced accumulated symptom rate and improved the survival rate caused by nasal inoculation of Flu to mice. In addition, we fractionated, analyzed and investigated functional substance of CE. In the results, we found that anti-Flu functional substance of CE would be aromatic compounds without anthocyanins. These results indicate that CE has utilizable components to prevent infection of Flu A type.