PURPOSE: To measure an antigenic substance in a body liquid with high sensitivity.

CONSTITUTION: In a method for measuring the amount of an antigenic substance, the amount of a labeling substance in an immunity complex obtained by bonding a monoclonal antibody (a first antibody) in which an insoluble solid is bonded to an antigenic substance as an object under test and which recognizes the antigenic substance, a monoclonal antibody (a second antibody) which recognizes the antigenic substance and which is a class or a subclass different from the first antibody and a monoclonal antibody (a third antibody) which is labeled by a labeling substance and which recognizes the class or the subclass of the second anti body. In the immunity-measuring method of the antigenic substance, monoclonal antibodies which are derived from animals of the same species are used for the first antibody, the second antibody and the third antibody.