PROBLEM TO BE SOLVED: To provide a technique developed to detect an influenza virus more highly sensitively than a conventional immunochromatography method and find whether the detected influenza virus is susceptible to a neuraminidase activity inhibitor or not.SOLUTION: Provided is a technique in which a neuraminidase activity inhibitor is used in vitro in combination with N-acetylneuraminic acid-luciferin and luciferase to determine the efficacy of the neuraminidase activity inhibitor against a detected influenza virus.