1. A method for measuring haemagglutinin of an influenza virus by a sandwich immunoassay method, said method comprising:extracting haemagglutinin from a sample containing the influenza virus by adding a cationic or anionic surfactant;
combining the extracted haemagglutinin with at least one lectin immobilized on a solid support and with a labeled anti-haemagglutinin antibody that undergoes antigen-antibody reaction with the haemagglutinin; and
measuring the haemagglutinin by sandwich immunoassay, wherein detection of the presence of haemagglutinin indicates the presence of influenza virus,
wherein the at least one lectin binds the haemagglutinin, but does not bind the anti-haemagglutinin antibody, and wherein the at least one lectin is Datura stramonium lectin, and
wherein the cationic and anionic surfactant is at least one selected from the group consisting of sodium dodecyl sulfate, lithium dodecyl sulfate, hexadecyltrimethylammonium bromide, hecadecyltrimethylammonium chloride and hexadecylpyridinium chloride.