It is intended to provide an immunoassay method which can be appropriately carried out on a chip. Namely, an immunoassay method which comprises forming an antigen-antibody complex, wherein an antigen (2) to be assayed binds to an antibody (3), in a reaction chamber (7), washing the reaction chamber (7) with a sample solution (8) and thus separating the antibody not binding to the antigen to be assayed. According to this method without resorting to a washing liquor free from an antigen to be assayed (for example, a protein) typified by Tris-HCl butter, a reaction chamber can be washed to such an extent that a signal reflecting the amount of the antigen to be assayed can be detected at an accuracy comparable to the level achieved by using the above-described washing liquor. Thus, it becomesunnecessary to supply a washing liquor from the outside to a chip or preliminarily provided a washing liquor on a chip so that the immunoassay can be easily carried out on a chip.