Abstract: An immunochromatography which comprises chromatographically moving a sample together with or followed by labelling fine particles in a chromatographic medium having at least one reaction site having immobilized thereat a reagent bindable to a substance to be detected in a sample, to contact the above sample and the labelling fine particles with the above reaction site, and detecting the substance to be detected by use of the phenomenon that when the above substance to be detected is present in the sample the labelling fine particles are specifically bound to the above immobilized reagent via the substance to be detected at the above reaction site, to thereby capture the substance to be detected on the chromatographic medium, characterized in that the above labelling fine particles are sensitized dyed particles obtained by sensitizing, with a material bindable to the above substance to be detected, labelling dyed particles obtained by dyeing latex particles of a synthetic high polymer, said labelling dyed part