An immunochromatoassay method that allows high detection sensitivity measurement. The method including the steps of: permeating an analyte solution that includes a visibly labeled second binding substance that specifically binds to a detection target substance into a test area of a chromatography medium provided with a first binding substance that specifically binds to the detection target substance, simultaneously with or after the permeation of the analyte solution into the test area, permeating a visual recognition aid solution into the chromatograph medium, the solution having a refractive index whose refractive index difference Δn from that of the chromatograph medium is −0.1≦Δn≦0.1, and visually observing the test area while the visual recognition aid solution is permeated in the test area.