(i) mixing a specimen with a nitrite solution to form a mixture;

(ii) allowing a solid acid reagent to come into contact with the mixture in a filtration step to extract a sugar chain antigen contained in the specimen;

(iii) adding the mixture to an immunochromatographic test piece, wherein the immunochromatographic test piece comprises a sample pad, a neutralizing reagent (basic reagent)-impregnated non-woven fabric, a label region comprising a dry pad, a detection region comprising an antibody-immobilized membrane, and an absorption band, and

(iv) allowing the mixture to flow through the test piece, thereby measuring the sugar chain antigen,

wherein the filtration step is carried out in a specimen adding device separated from an immunochromatographic test piece, the specimen adding device comprising a vessel portion and a lid portion having a specimen adding nozzle and further comprising (i) a porous material impregnated with tartaric acid and (ii) a filter;

wherein allowing a solid acid reagent to come into contact with the mixture to extract a sugar chain antigen is carried out in the filtration step;

wherein the solid acid reagent is neutralized by the neutralizing reagent (basic reagent)-impregnated non-woven fabric; and

wherein the solid acid reagent is tartaric acid,

wherein the dry pad comprises antibody-immobilized colored polystyrene particles.