1. A measurement method for measuring a displacement of a surface of an object from an image of a two-dimensional grid image on the surface of the object taken by a camera, comprising the steps of: taking an image of a two-dimensional grid image on a surface of an object with a camera in such a state that the direction of camera pixels is adjusted towards said image of the two-dimensional grid image before displacement, and at the same time one period of said image of the two-dimensional grid image in the x direction is adjusted to Nx pixels of the camera and one period in the y direction is adjusted to Ny pixels of the camera, where Nx and Ny are integers greater than 2; obtaining a phase from said image of the two-dimensional grid image for the frequencies in the x direction and the y direction that correspond to one period of said grid in said image of the two-dimensional grid image; and finding a displacement of the surface of the object from the phase.