Immunochromatography method for assaying a target antigen comprising: a mixing step of mixing an antigen-containable specimen and modified magnetic particles, which are magnetic particles modified with a monoclonal antibody X having a specific affinity to the antigen, to obtain a mixture containing magnetic particle composite bodies which are composite bodies of the antigen and the modified magnetic particle; a collection step of collecting magnetic particles in the mixture containing the magnetic particle composite bodies using magnetism; a dissociation step of dissociating the modified magnetic particles from the magnetic particle composite body to obtain an antigen-containing solution by mixing the magnetic particles collected in the collection step with a dissociation solution which is an acidic or alkaline solution, then removing at least part of the dissociated modified magnetic particles; a neutralization step of neutralizing the antigen-containing solution using a neutralization solution to obtain a neutralized antigen-containing solution; a spreading step of spreading gold particle composite bodies on an insoluble carrier in an immunochromatograph having a reaction site at which a monoclonal antibody Z capable of binding to an antigen in the neutralized antigen-containing solution has been immobilized, in a state where the gold particle composite bodies which are composite bodies of the antigen and modified gold particles which are gold particles modified with a monoclonal antibody Y capable of binding to the antigen are formed; a capturing step of capturing the gold particle composite bodies at the reaction site of the insoluble carrier; and a silver amplification step of silver-amplifying the gold particle composite body captured in the capturing step, wherein the epitope of the monoclonal antibody X and the epitope of the monoclonal antibody Y are different from each other, and the epitope of the monoclonal antibody X and the epitope of the monoclonal antibody Z are different from each other. a mixing step of mixing an antigen-containable specimen and modified magnetic particles, which are magnetic particles modified with a monoclonal antibody X having a specific affinity to the antigen, to obtain a mixture containing magnetic particle composite bodies which are composite bodies of the antigen and the modified magnetic particle; a collection step of collecting magnetic particles in the mixture containing the magnetic particle composite bodies using magnetism; a dissociation step of dissociating the modified magnetic particles from the magnetic particle composite body to obtain an antigen-containing solution by mixing the magnetic particles collected in the collection step with a dissociation solution which is an acidic or alkaline solution, then removing at least part of the dissociated modified magnetic particles; a neutralization step of neutralizing the antigen-containing solution using a neutralization solution to obtain a neutralized antigen-containing solution; a spreading step of spreading gold particle composite bodies on an insoluble carrier in an immunochromatograph having a reaction site at which a monoclonal antibody Z capable of binding to an antigen in the neutralized antigen-containing solution has been immobilized, in a state where the gold particle composite bodies which are composite bodies of the antigen and modified gold particles which are gold particles modified with a monoclonal antibody Y capable of binding to the antigen are formed; a capturing step of capturing the gold particle composite bodies at the reaction site of the insoluble carrier; and a silver amplification step of silver-amplifying the gold particle composite body captured in the capturing step, wherein the epitope of the monoclonal antibody X and the epitope of the monoclonal antibody Y are different from each other, and the epitope of the monoclonal antibody X and the epitope of the monoclonal antibody Z are different from each other.