A criterion in non-destructive test method for fracture of reinforcing steel bars by measuring magnetic flux density was established. The steel bar is ferromagnetic. With being magnetized with a magnet from on the surface of concrete cover, disorder of magnetic flux appears near the fractured part. The diagnosis could be done at the bent part of the reinforcing steel bar clearly using proposed criterion with taking degree of the disorder into account in model cage assemblies of steel bars. The criterion was also applicable for existing reinforced concrete beam members in road bridge substructures.
