An inspection system uses a scanning electron microscope that detects a high-precision electron beam image, and at the same time removes restrictions for a low sampling rate. A sampled signal is obtained by sampling an analog brightness signal generated by a secondary electron detector at a predetermined sampling rate, and contiguous digital values contained in the sampled signal are added on an N by N digital value basis to generate a digital brightness signal whose frequency is equal to 1/N of the sampling frequency. Each, digital value contained in the digital brightness signal is divided by N to generate a digital signal made of digital values having a number of bits equal to that of the sampled signal and to generate an image signal in which each digital value of the digital signal forms one pixel data.