A square-shaped beam of charged particles is passed over a registration mark, which is formed by a depression in the surface of a semiconductor wafer. When the beam passes over one edge of the mark, a positive peak signal is produced while a negative peak signal is produced when the beam passes over the other edge of the mark. These positive and negative signals are compared with positive and negative threshold signals in comparators with an output signal being produced from each of the comparators when its threshold signal is crossed. This enables location of each of the edges of the mark to be determined. The positive and negative threshold signals are set for each of the areas of the wafer having one of the marks since different signal baseline voltages are produced by different areas of the wafer.