A distortion evaluating apparatus which can quantitatively evaluate distortion in a measurement object surface is provided. A distortion evaluating apparatus evaluates distortion based on three-dimensional measurement data obtained from a measurement object surface. The apparatus includes a secondary differential component adapted for effecting a secondary differential operation on two-dimensional measurement data of a cross section of the measurement object surface indicative of unevenness therein, thus obtaining curvature data of the cross section, a permissible range setting component adapted for setting a permissible range for the curvature data, based on range of an upper limit value and a lower limit value from a reference value, and a distortion data extracting means component adapted for extracting a portion of the curvature data exceeding the set permissible range as distortion data indicative of the distortion in the cross section.