The present invention provides an immunological assay suitable to be carried out on a chip. After an antigen-antibody complex in which an antigen 2 to be measured and an antibody 3 have bound to each other was obtained in a reaction chamber 7, the amount of the antigen-antibody complex or the antibody that has not bound to the antigen to be measured by using a measurement solution obtained by using the sample solution 8 as a solvent. According to the present invention, the measurement solution for detecting the signal that reflects the amount of the substance to be measured can be obtained without using a buffer solution free from a substance to be measured, such as protein. Thus, it is not necessary to supply a buffer solution from the outside of the chip or to allow the buffer solution to be retained on the chip beforehand. Accordingly, an immunological assay can be carried out easily on a chip.