The invention relates to an immunochromatographic device which contains a nitrous acid compound and an organic acid or an organic acid derivative and which is for detecting a detection target in an analyte wherein a sample droplet-receiving member, a labeling substance-holding member, a chromatography medium member and an absorption member are arranged in a manner that a sample develops in this order and wherein a part containing the nitrous acid compound and a part containing the organic acid or the like are at upstream positions from the labeling substance-containing part, and the part containing the nitrous acid compound and the part containing the organic acid or the like are not substantially in contact with each other in the thickness direction.