Method for eliminating prozone phenomenon in immunoassay reagent

The present invention addresses the problem of providing an immunochromatograph detection method and immunochromatograph kit in which the prozone phenomenon, which occurs in a sandwich method in a specimen in which too much antibody is present, is suppressed or eliminated. The abovementioned problem is overcome by a configuration in which a non-ionic, hydrophilic surfactant having an HLB value of 14-18 is retained in a sample addition part of an immunochromatography device, and a membrane having a high flow speed is used as the membrane used in a chromatograph medium.