A method of diagnosing metabolic bone diseases, especially osteoporosis and arthrosis characterized by determining the concentration of osteoclastgenesis inhibitory factor (OCIF) in humor. Monoclonal antibodies recognizing equally both of monomer type and dimer type of OCIF. Monoclonal antibodies recognizing selectively dimer type of OCIF. And to provide an assay kit for determination of OCIF concentration comprising the aforementioned two antibodies recognizing different epitope of OCIF and having high affinity showing dissociation constant of less than 2×10−7 M with antigen. It is useful for a method of diagnosing metabolic bone diseases, especially osteoporosis and arthrosis or for an assay reagent for research thereof.