[en] In a nuclear power plant, where many piping are connected to vibratory sources like pumps, it is important to estimate vibrational stress efficiently to keep the plant operating safely. The purpose of this study is to develop a method to measure vibrational stress by multiple laser displacement sensors. The effectiveness of the method is discussed applying the beam theory and the applicability of the method is verified by vibration tests. First, an error analysis based on the beam theory was examined; thus the feasibility of the method was confirmed as a whole estimation process for vibrational stress. Next, the stresses calculated by the proposed method were compared with the result obtained from strain gauges for the vibration tests of a plate applying sinusoidal and random waves. The stress calculated by the method had the errors of about ±5MPa compared with the stress obtained from strain gauges for the vibration tests applying sinusoidal wave. And the tendency of vibrational stress obtained from strain gauges were in good agreement with the stress calculated by the method for the vibration test applying random wave. (author)