The present invention provides a means and a method for specifically measuring a substance such as a small substance with high sensitivity by a sandwich method. Specifically, the present invention provides an antibody capable of specifically binding to an affinity complex and a method of measuring of the affinity complex comprising measuring the affinity complex using the antibody capable of specifically binding to the affinity complex. The antibody of the present invention may be a full-length antibody. The antibody of the present invention may also have a region derived from an immunoglobulin from an animal having an ability of gene conversion (e.g., a complementarity-determining region, a framework region, or a variable region). Examples of at least one factor that constitutes the affinity complex include a small substance or a protein (e.g., antibody).