1. A linear estimation method for obtaining a three-dimensional position of an object point with a plurality of cameras, comprising: a first step of acquiring images of a plurality of reference points being located on known positions in said three-dimensional space by said plurality of cameras and obtaining coordinates of projected points thereof on respective said images; and a second step of assuming a plurality of affine cameras having linear relation between said three-dimensional space and images, calculating coordinates of affine camera projected points corresponding to how said reference points are projected by said affine cameras and correcting the coordinates of said obtained projected points to be consistent with the coordinates of said affine camera projected points; wherein said second step regards coefficients of a second order polynomial of the coordinates of said affine camera projected points as parameters for correction.