In March 2017, the damage occurred at the first valve weld of the steam generator A of a nuclear power plant, resulting in leakage of the coolant. In June 2008, the steam generator B experienced damage at the same position. It is considered that the cause of high vibration and damage of the steam generator drain pipe is due to the structural vibration of the main body and the acoustic resonance of the vortex pulsation and the acoustic frequency generated in the mouse hole of the branch pipe. It is not possible to calculate the passing velocity required to calculate the vortex pulsating frequency because it can not confirm the exact shape information of the mouse hole. However, if it is assumed that the original model was affected in the acoustic resonance, it will be out of the resonance influence by the design change. As a result of vibration response analysis, compared with the allowable stress specified in ASME code, the original model was unsatisfied, while the changed model was satisfactory. Therefore, it is expected that the vibration response characteristics will be greatly improved after the design change, so that even if unexpected external vibration occurs, the soundness can be maintained.