A system for generating an error image, the system including a three-dimensional measuring instrument, a CAD system, an arithmetic processing unit and an output device, wherein the arithmetic processing unit is inputted with measurement points obtained by measuring a measured surface of a workpiece by the three-dimensional measuring instrument and free-form surface data which is design data of the workpiece, and in which the arithmetic processing unit is configured to calculate error data between the measurement points and the free-form surface data, to generate the error image using the error data, and to output the error image to the output device, wherein the arithmetic processing unit comprises: a first point group/error calculating unit for calculating a point group on a surface parametric space of the free-form surface data corresponding to the measurement points, as a first point group, and for calculating error data between each of the measurement points and design points on the free-form surface data corresponding to the measurement points; a first triangulating unit for triangulating the first point group to generate first triangular facets and calculate a convex hull of the first point group; a second point group generating/error interpolating unit for outputting a second point group distributed in the calculated convex hull on the surface parametric space and for obtaining error data of each of points of the second point group by interpolation calculation from the error data corresponding to the first point group which is a vertex of the first triangular facets in which each of the respective points of the second point group is included; a second triangulating unit for re-triangulating a point group including the first point group and the second point group to generate second triangular facets; and a three-dimensional image generating unit for generating the error image using the second triangular facets and error data of a vertex of the second triangular facets and outputting the error image to the output device. a first point group/error calculating unit for calculating a point group on a surface parametric space of the free-form surface data corresponding to the measurement points, as a first point group, and for calculating error data between each of the measurement points and design points on the free-form surface data corresponding to the measurement points; a first triangulating unit for triangulating the first point group to generate first triangular facets and calculate a convex hull of the first point group; a second point group generating/error interpolating unit for outputting a second point group distributed in the calculated convex hull on the surface parametric space and for obtaining error data of each of points of the second point group by interpolation calculation from the error data corresponding to the first point group which is a vertex of the first triangular facets in which each of the respective points of the second point group is included; a second triangulating unit for re-triangulating a point group including the first point group and the second point group to generate second triangular facets; and a three-dimensional image generating unit for generating the error image using the second triangular facets and error data of a vertex of the second triangular facets and outputting the error image to the output device.