A focused ion beam apparatus enables an ion beam to be focused highly accurately on a sample at the beam spot position of the case of the absence of magnetic field without causing isotope separation of the ion beam on the sample, even when there is a magnetic field on the ion beam optical axis or the magnetic field fluctuates. The focused ion beam apparatus comprises a corrective magnetic field generating unit 10 disposed on the optical axis of the ion beam 3 for correcting the deflection of the ion beam 3 due to an external magnetic field. The corrective magnetic field generating unit 10 includes pole-piece pairs 26A and 26B, each of which having two pole pieces 26a and 26b or 26c and 26d that are adjacent to each other with a gap d. The pole-piece pairs 26A and 26B are disposed opposite to each other with a gap g (>d) across the optical axis of the ion beam 3. Each of the pole pieces 26a to 26d is wound with an internal coil 29, and the pole-piece pairs 26A and 26B are each wound with an external coil 30 in such a manner as to surround the internal coils 29.