A pattern inspection apparatus is designed to quickly and accurately perform an inspection of an inspection sample, such as a mask or a wafer or the like by irradiating electron beams onto the inspection sample and detecting secondary or backscattered electrons reflected from the surface of the inspection sample and/or transmitted electrons passing through the inspection sample. The pattern inspection apparatus includes an electron beam generator including at least one electron gun for generating at least one electron beam irradiating onto the surface of the inspection sample. A movable support is provided for supporting the inspection sample. The apparatus also includes a detector unit having a plurality of electron detecting elements for detecting electrons containing information related to the construction of the inspection sample and a detection signal processor for processing simultaneously or in parallel formation the outputs of the electron detecting elements of the detector. Also, when a plurality of electron beams are used for simultaneous irradiation of the inspection sample, the pattern inspection apparatus includes a mechanism for avoiding interference between the reflected electrons of the adjacent electron beams.