The present invention provides a method of pretreating a specimen, which allows measurement according to an immunoassay to be carried out on a specimen from nasal secretion while preventing non-specific reactions. According to this method, the specimen from nasal secretion is treated with a protease beforehand and then an immunoassay is performed. As the protease, it is preferable to use semi-alkaline protease (EC 3.4.21.63). Furthermore, it is preferable that a substance to be pretreated by the pretreatment method according to the present invention is an influenza virus contained in the specimen from nasal secretion. The immunoassay preferably is an immunoagglutination assay. Examples of the immunoagglutination assay include a turbidimetric immunoassay, a latex turbidimetric immunoassay, and a latex agglutination assay that is performed on a slide glass.