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This paper describes a novel and simple method for measuring in-plane Poisson's ratio of film materials. We designed on-chip pure-bending test specimen where pure-bending deformation can be produced via torsion bar by application of normal load to loading lever. During pure-bending, the interference pattern of a family of hyperbola, corresponding to the contour lines of a film specimen in the out-of-plane direction, is observed with optical interferometer. In-plane Poisson's ratio of a film specimen can be obtained from only the angle of asymptotes of hyperbola consisting of the interference lines, regardless of other material constants. The measured Poisson's ratio of single crystal silicon (SCS) specimen was 0.063 on average, in close agreement within 2 % deviation from analytical value.