Abstract: The disclosed apparatus and method diagnose deterioration of a steel-reinforced concrete smokestack by taking its infrared ray photograph, thermographically processing the photograph into an outside surface temperature distribution diagram so as to detect a defect as a singular point on the diagram, measuring the circumferential length (L) of the defect from the diagram, finding concrete crack width (W) at the defect based on outside surface temperature difference between sound portion of the smokestack and the defect, and calculating reduction of the strength of the smokestack wall structure due to the defect by using the circumferential length (L) and the concrete crack width (W) of the defect.