Abstract: An immunoassay method and a kit which comprises a combination of a test piece for immunochromatography and a developing solvent, by which a target substance is detected accurately in a short period of time while a preventing a nonspecific reaction. The above method is to immunologically measure the target substance by using the test piece for immunochromatography which comprises a membrane carrier having a first antibody or a first antigen capable of binding specifically to the target substance immobilized thereon, an insoluble carrier having a second antibody or a second antigen capable of binding specifically to the target substance immobilized thereon and a porous separation matrix capable of cell separation, wherein the porous separation matrix is impregnated with a surfactant in advance, and the insoluble carrier bound specifically to the target substance through the second antibody or the second antigen is developed with a developing solvent containing mannitol.