The present invention provides an immunological assay suitable to be carried out on a chip. After an antigen-antibody (2, 3) complex in which an antigen to be measured and an antibody have bound to each other was obtained in a reaction chamber (7), the reaction chamber (7) is washed using a sample solution (8) and thereby the antigen-antibody complex and the antibody that has not bound to the antigen to be measured are separated from each other. According to the present invention, the reaction chamber can be washed to an extent that allows a signal reflecting the amount of the antigen to be measured to be detected with accuracy comparable to the case of using a wash solution without using a wash solution that is typified by Tris-HCL buffer that does not contain any antigens to be measured such as protein. Thus, it is not necessary to supply a wash solution from the outside of the chip or to allow a wash solution to be retained on the chip beforehand. Accordingly, an immunological assay can be carried out easily on a chip.