1. A scanning electron microscope comprising: an electron optics for scanning an electron beam on a predetermined region of a standard specimen whose pattern dimension or pattern pitch is known as a nominal value; detection means for detecting secondary electrons or backscattering electrons generated under the scan of the electron beam; and operation means for calculating a pattern dimension or pattern pitch in said predetermined region, wherein said operation means includes means for forming an image from information of secondary electrons or backscattering electrons detected by said detector, means for forming a plurality of regions from said image, means for measuring a pattern dimension or pattern pitch in each of said plurality of formed regions, and means for calculating a scanning distortion amount of said electron beam from distribution of the pattern dimension or pattern pitch.