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 substance 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-concentrated solution by mixing the magnetic particles collected in the collection step with a dissociation solution which is an alkaline or acidic solution, where an amount of the dissociation solution is smaller than the antigen-containable specimen, then removing at least part of the dissociated modified magnetic particles, wherein the dissociation solution contains NaOH or HCl; a neutralization step of neutralizing the antigen-concentrated solution using a neutralization solution to obtain a neutralized antigen-concentrated solution, wherein the neutralization solution contains HCl and at least one selected from the group consisting of tricine, Tris, HEPES, acetamidoglycine, glycinamide, and vicine, or contains NaOH and at least one selected from the group consisting of tricine, Tris, HEPES, acetamidoglycine, glycinamide, and vicine; a spreading step of spreading gold particle composite bodies on an insoluble carrier_in an immunochromatograph having a reaction site at which a second binding substance capable of binding to an antigen in the neutralized antigen-concentrated 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 first binding substance 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. a mixing step of mixing an antigen-containable specimen and modified magnetic particles, which are magnetic particles modified with a substance 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-concentrated solution by mixing the magnetic particles collected in the collection step with a dissociation solution which is an alkaline or acidic solution, where an amount of the dissociation solution is smaller than the antigen-containable specimen, then removing at least part of the dissociated modified magnetic particles, wherein the dissociation solution contains NaOH or HCl; a neutralization step of neutralizing the antigen-concentrated solution using a neutralization solution to obtain a neutralized antigen-concentrated solution, wherein the neutralization solution contains HCl and at least one selected from the group consisting of tricine, Tris, HEPES, acetamidoglycine, glycinamide, and vicine, or contains NaOH and at least one selected from the group consisting of tricine, Tris, HEPES, acetamidoglycine, glycinamide, and vicine; a spreading step of spreading gold particle composite bodies on an insoluble carrier_in an immunochromatograph having a reaction site at which a second binding substance capable of binding to an antigen in the neutralized antigen-concentrated 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 first binding substance 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.