A measurement unit for tensile or compressive stress can includes a CCD camera for detecting an interference light, the interference light being formed with a measurement beam from a measured region and a reference beam from a reference mirror. A first objective lens can have the reference mirror. An image processing apparatus can measure the three-dimensional shape of the measured region from the position of the first objective lens at which the interference light provides the maximum contrast and can measure the distance between two gauge points on the basis of the three-dimensional shape. When strain is generated on a micromaterial, the strain against the measured tensile stress is measured on the basis of the tensile stress and the distance between the two gauge points.