1. A dimension measuring scanning electron microscope system comprising: an input means for entering design pattern data on a mask pattern designed by taking into consideration optical proximity effect and to be formed on a photomask; a SEM image forming means for forming a SEM image of a resist pattern formed by exposing a resist film formed on a surface of a substrate through the photomask provided with the mask pattern formed by taking into consideration optical proximity effect to exposure radiation; a simulating means for determining a simulated circuit pattern that is expected to be formed on a substrate through simulation using the mask pattern defined by the design pattern data entered by the input means; a superposing means for superposing the SEM image of the resist pattern formed by the SEM image forming means and the simulated circuit pattern formed by the simulating means; geometrical feature extracting means for extracting one- or two-dimensional geometrical features indicating differences between the SEM image and an image of the simulated circuit pattern superposed on the SEM image; and an evaluating index calculating means for calculating evaluation indices for evaluating the correction of the optical proximity effect of the mask pattern formed on the photomask by taking into consideration the optical proximity effect by using information about the one- or two-dimensional geometrical features provided by the feature extracting means.