Abstract: Provided is an immunochromatography method that enables highly sensitive detection by reducing the background density of a non-measurement site in the immunochromatography method. The immunochromatography method includes developing a complex of a substance to be tested and a labeling substance containing a metal modified with a first binding substance for the substance to be tested on an insoluble carrier in the presence of a surfactant having a steroid skeleton while the substance to be tested and the labeling substance form the complex; and detecting the complex of the substance to be tested by capturing the substance to be tested and the labeling substance on a detection site of the insoluble carrier containing a second binding substance for the substance to be tested or a substance that can bind to the first binding substance for the substance to be tested.