Disclosed is an operation for an optical system which achieves observation of focused ion beam processing equivalent to that in a case wherein a sample stage is tilted mechanically. In a focused ion beam optical system, an aperture, a tilting deflector, a beam scanner, and an objective lens are controlled so as to irradiate an ion beam tilted to the optical axis of the optical system, thereby achieving thin film processing and a cross section processing without accompanying adjustment and operation for a sample stage. The thin film processing and the cross section processing with a focused ion beam can be automated, and yield can be improved. For example, by applying the present invention to a cross section monitor to detect an end point, the cross section processing can be easily automated.