A novel structural diagnosis method for reinforced concrete members is proposed to detect concrete cracks and to evaluate the amount of reinforcing steel. In the proposed method, the soundness of the member is diagnosed from the wave profile of the monitored natural frequency with thermal loads. The principle of the method is experimentally investigated in the paper, which lies in that the natural frequency transition of a concrete member differs by the thermal stress distribution in the member section, and that the thermal stress distribution also differs by the occurrence of the concrete crack or the amount of reinforcing steel.