An automatic method of measuring surface strain by electro-optical techniques is described. It is a real-time and point-by-point method in which a specimen surface is illuminated by a thin laser beam and the displacements of the speckle pattern resulting from the surface deformation are detected by linear image sensors followed by a microcomputer. The displacements are calculated as the peak positions of the cross-correlation functions of image sensor signals before and after the deformation. By adopting a differential system consisting of two image sensors positioned symmetrically about the surface normal we can eliminate the effect of rigid body motion. The sensitivity depends on the surface to sensor distance, while the gauge length is given by the diameter of the laser spot. These relations derived from physical optics are verified by experiments using a metal plate specimen subject to tensile strain that is monitored by an electrical resistance strain gauge. The minimum strain value measured was a few ten microstrains with a gauge length of 1 mm. We could also measure thermal strain by this method.