In this study, a new diagnosis approach for a beam structure is proposed to increase the robustness of the stepwise primary diagnosis method. The distinguishing feature of this method is that the mathematical model is modified based on the difference of the response between the measurement and the simulation. To this end, the response ratio is defined between the measured and simulated response under the normal condition, and the mathematical model is modified by using the response ratio to adjust it to the measured property. Using the random excitation test, we can identify the location of abnormality at various excitation frequencies and obtain the final result of the abnormal location based on its mean value with a standard deviation. We checked the feasibility of the proposed method using the experimental data of a free-free uniform beam excited at the center. An additional mass was considered as an example of abnormality. As a result, the location and the cause of abnormality can be identified for five levels of abnormality by using the proposed method. The results indicate that the proposed method is a feasible way to diagnose the practical structure of a beam.