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In this paper, a simple experimental technique for the measurement of the Poisson's ratio of a small-sized specimen is described. When two equal and opposite bending moments are applied to a rectangular solid structure, the structure deforms like a saddlelike shape on the basis of the linear elastic theory. The contour line on the structure middle portion is represented as a family of hyperbolas, and the angle between the asymptote and the structure's width direction is related to the Poisson's ratio. We applied the principle to a microspecimen. Based on Searle's analysis, the specimen was designed by means of finite element analysis. Single crystal silicon (SCS) was chosen as the first target material, and the specimen was manufactured by conventional micromachining technologies. The test equipment with a loading mechanism and a contour line measurement function using an optical interference method was developed. After precise alignment between the loading point and the center of the loading lever, normal loading was applied. From the obtained optical interference patterns, the mean angle of 14.5° was obtained in the penetration depth ranging from 25 to 100 μm. The angle provided the Poisson's ratio, 0.068, which was in very good agreement with the ideal value of the in-plane Poisson's ratio, 0.064, for SCS(001). The developed technique has the potential for evaluating the Poisson's ratio of small-sized specimens.