Abstract: The present invention relates to a method for selectively detecting a multimer type multimer-forming polypeptide in a biological sample, the method comprising: (a) bringing the biological sample into contact with an agglutination reaction inducing agent to induce the formation of an aggregate in an analysis target, the agglutination reaction inducing agent being a particle in which a specific antibody is surface-bonded with the multimer-forming polypeptide; (b) obtaining an image with respect to the aggregate of step (a); and (c) analyzing a size or a shape of the aggregate by using the image. Step (a), step (b), or steps (a) and (b) are performed on a microchip having a microchannel. The image analysis is performed using a coefficient according to the size of the aggregate in a predetermined volume provided by the microchannel.