A method and apparatus for correcting drift of the beam irradiation position during automated FIB (focused ion beam) processing with a reference image-setting unit, an image read-in unit for reading in images of the reference image region during the FIB processing, an arithmetic-and-control unit for finding the direction and amount of image deviation between the subsequent images, and a beam deflection system-adjusting unit for correcting the beam deflection system by correcting the image deviation based on the deviation in response to the output from the arithmetic-and-control unit. The arithmetic-and-control unit optimizes the brightness or contrast of the reference image.