Disclosed herein is an immunoassay method for detecting the detection target antigen in an analyte through extraction with an extraction agent. The method is characterized in that the detection is performed in the presence of a cyclic oligosaccharide. The method is also characterized in the use of an immunochromatography kit for detecting gram-positive bacteria in an analyte, and that is configured from an analyte dilution solution, a sample dropping part, an antigen extracting part, a labeling substance retaining part, an immunochromatography medium having a detection part, and an absorption part, and in which an organic acid is retained in the antigen extracting part. The immunochromatography kit contains a cyclic oligosaccharide and a nitrite compound by containing at least one of a cyclic oligosaccharide, a nitrite compound, and a mixture thereof in the analyte dilution solution and/or the sample dropping part.