The present inventors attempted to produce antibodies that neutralize the activity of a substance having an activity of functionally substituting for FVIII to be used for the method of measuring the reactivity of FVIII in the presence of a substance having an activity of functionally substituting for FVIII. As a result, the inventors discovered that by using the produced antibodies, FVIII activity in the plasma of a hemophilia A patient can be evaluated accurately by one-stage clotting assay based on APTT, and also that FVIII inhibitor titer in the plasma of a hemophilia A patient carrying a FVIII inhibitor can be evaluated accurately by Bethesda assay based on APTT.