The fault detecting apparatus comprises an electro optical lens-barrel having a rectangular cathode (101), three four-pole lenses (117, 119, 121), and a deflector (129). The four-pole lenses are controlled to form such a rectangular beam that a ratio of a reduction ratio at the sample surface of an electron beam locus along a longitudinal direction of the rectangular cathode to a reduction ratio at the sample surface of an electron beam locus along a lateral direction of the rectangular cathode becomes equal to a ratio of a length to a width of the rectangular cathode and in addition a width of the beam is equal to a required minimum fault detection width. Further, the deflector (129) is controlled by a deflection controller (130) in such a way that the rectangular beam can be scanned (raster scanning) by moving the rectangular beam at every scanning stroke corresponding to the minimum fault detection width in both the longitudinal and lateral directions of the rectangular beam.