A phase of a specific frequency is found by carrying out a two-dimensional Fourier transformation on an image of a two-dimensional grid image on the surface of an object taken by a camera, and the displacement of the surface of the object is measured from the phase. As a result, measurement that is strong against noise can be carried out without projecting a grid having a brightness distribution of precise cosine waves. In addition, the process is simple and the number of pixels used for the measurement is smaller than that in the sampling moire method. The displacement can be found at a high speed.