This paper deals with an automatic linewidth measurement method of photoresist pattern linewidths with high accuracy, which we call ‘‘the linear regression method.’’ In the linear regression method, the secondary electron intensity profile at the photoresist pattern edge is approximated by two lines. One is the average line of the secondary electron intensity profile on the substrate and another is the slope line corresponding to the slope of the photoresist edge. The cross point of the two lines is defined as the pattern edge. The linewidths obtained by this method agree with the linewidths obtained by the cross sectional SEM images which are defined as true values, within 0.009 μm for various photoresist slope angles and within 0.002 μm for variation of the underlying materials. A practical pattern linewidth measuring apparatus which employs a high speed measurement time of 2.34 s is also described.