PROBLEM TO BE SOLVED: To find a naturally-derived substance having an anti-influenza virus action free from side effects, and to provide a new anti-influenza virus agent containing the substance.SOLUTION: There is provided 5-hydroxymethyl-2-furfural organic acid ester, isolatedly purified from a Japanese plum extract having an excellent anti-virus action against human type A influenza virus. Further, the substance has the two actions of: a neuraminidase inhibitory action for new type influenza virus strain (A/Narita/1/2009(H1N1)) causing the present global great fashion； and a hemagglutination inhibition action by the virus. Thereby a naturally-derived new anti-influenza virus agent free from side effects can be provided.
