Abstract: An automatic focusing apparatus incorporated in a scanning electron microscope. The focusing apparatus permits accurate focusing even if a very uneven specimen surface is observed. The apparatus has a secondary electron detector. The output signal from this detector is supplied to a peak-holding circuit via a low-pass filter. The greatest peak value of the signal is detected and stored in a memory whenever the electron beam makes a scan at each different focal length of the objective lens. This processing is performed concurrently with integration of the output signal from the detector. This set of operations is repeated for all focal lengths. Data in the memory are compared with each other by a comparator, and the greatest peak value is found. Data about the focal length providing the greatest peak value is supplied as a reference signal to another comparator. In response to this data, data about several focal lengths close to this focal length are read from the memory and compared with each other.