Speckle interferometry is a useful optical deformation measurement method for objects with rough surfaces. This method can measure not only the out-of-plane but also the in-plane deformation of these objects. Recently, the method has been used to measure three-dimensional (3-D) deformations at a high resolution. Many kinds of 3-D measurement methods based on speckle interferometry have already been proposed. However, the parameters in the sensitivity matrices of these conventional methods are not generally the same in each of the 3-D. Methods for which the sensitivity is not the same in the 3-D cannot be employed as an exact 3-D measurement method because the measurement uncertainty is different in each direction. This paper proposes a method where the sensitivity is the same in 3-D. An optical system was set up based on the concept of the proposed method. A method based on vector analysis is also proposed for detecting the deformation in an arbitrary section of the object. Finally, the 3-D deformation from the buckling of a mechanical beam was measured using in new speckle interferometry.