Means for enabling an immunoassay with a sufficient sensitivity in an immunoassay for measuring influenza virus in a sample using influenza virus M1 protein as an antigen is provided. A sample processing method in an immunoassay for influenza virus, which method comprises, in an immunoassay for influenza virus using an antibody which undergoes antigen-antibody reaction with influenza virus matrix 1 protein, or an antigen-binding fragment thereof, bringing a sample containing influenza virus into contact with a sample processing liquid containing a surfactant having at least one group selected from the group consisting of palmityl, stearyl, and oleyl, is provided.