A strain distribution measuring system using a phase-shifting speckle shearing interferometer is proposed. A strain is obtained by the spatial differentiation of the in-plane component of the deformation on the object surface. In this interferometer, the spatial differentiation is performed optically by the interference with two sheared images formed by the diffraction grating in the imaging arrangements. The phase change of the interfering speckle between before and after deformation corresponds to the spatial differentiation of the deformation, i.e., the strain of the object surface. Moreover, in order to measure the phase change accurately, a 4-step phase-shifting method is adopted in our method. The phase-shifting is achieved by moving a diffraction grating. Hence two diffraction gratings are used for spatial shearing and a phase-shifting. As the phase change is calculated by a 4-step phase-shifting method at every point of object surface, the strain distribution can be obtained. The strain is the spatial differentiation of the in-plane component of the deformation, so this component should be measured. In order to measure this component, we illuminate the object surface from two directions which are symmetric with respect to the normal of the object surface. In this method, the calculation of the phase change is performed every illuminating directions independently, so two independent laser sources are used for two illuminating lights. The experimental result measuring the strain of the cantilever shows the validity of this measuring system. Using this system, a miniaturization and a practical application of the measuring system is anticipated.