An agglutination immunoassay method and apparatus therefor in which there are conducted the steps of contacting, in a container, a test sample suspected of containing a desired analyte and a reagent comprising sensitized magnetic-material containing particles capable of reacting with and binding to a desired analyte; precipitating the particles on the bottom of the container by the application of magnetic force; allowing the container to stand at an inclination so as to cause the particles to flow along the bottom of the container to form a developed pattern of the particles; obtaining a representative length of the developed pattern of the particles by an imaging device; and detecting the presence or absence of an immune reaction from the representative length of the pattern of the particles which has flowed from the bottom of the container, thereby detecting the presence or absence of the desired analyte in the test sample.