Immunoassay method and immunochromatographic kit

An immunochromatographic kit for detecting an antigen of a detection target in an analyte with a detection reagent capable of binding the antigen,

• the immunochromatographic kit comprising:
• a detection reagent;
• an analyte diluting solution;
• and
• an immunochromatographic device;
• wherein the immunochromatographic device includes:
• a sample dropping section;
• an antigen extracting section;
• a label-substance retaining section;
• a chromatographic medium having a detection section;
• and
• an absorbing section;
• wherein the analyte diluting solution and the sample dropping section contains a nitrite salt;
• wherein the antigen extracting section contains a heterocyclic compound having at least one skeleton selected from the group consisting of a five- or six-membered ring cyclic ester, a five- or six-membered ring cyclic amide, and a five- or six-membered ring cyclic imide;
• wherein the detection target comprises a gram-positive bacterium;
• wherein the antigen is a group-specific polysaccharide present in the cell wall of the gram-positive bacterium;
• wherein the detection reagent is capable of specifically binding said antigen;
• and
• wherein the kit contains a cyclic oligosaccharide.