1. A semiconductor inspection apparatus having a length measurement function for performing size measurement between a measurement start point and an end point of a scan image obtainable by scanning a focused electron beam on a wafer, said apparatus comprising: means for emitting an electron beam having an energy capable of penetrating a part of a workpiece and reaching an unexposed portion with respect to the electron beam; means for generating a scan image based on a secondary signal generated by irradiation of the electron beam; means for detecting, from a signal thus obtained, position information of a pattern to be inspected and a signal intensity; means for generating a stereoscopic model of the pattern to be inspected; means for using the position information and the signal intensity thus detected to calculate a feature quantity of a shape of the pattern to be inspected; means for constructing a stereoscopic structure from the feature quantity of the pattern calculated; and means for displaying a three-dimensional structure of the stereoscopic structure thus constructed.