Abstract: The present invention enables to firmly assemble a steel-frame structure. To form a steel-frame structure or to join two steel members being reinforcing members thereof, a slip-proof surface having one or plural concentric higher parts and grooves is formed on the joined surface of one side or both sides of the steel members, and these two steel members are clamped by a connecting member that passes through connecting holes in the state where the first and the second slip-proof surfaces are mutually engaged or not engaged. Thus, joining force between the joined surfaces is increased.