A focused charged particle beam apparatus including an aberration corrector, capable of finding the absolute value of the aberration coefficient at high speed, and capable of making high-accuracy adjustments at high speed. A deflection coil tilts the input beam relative to the object point, and measures the defocus data and aberration quantity at high speed while the beam is tilted from one image, and perform least squares fitting on these results to find the absolute value of the aberration coefficient prior to tilting the beam, and to adjust the aberration corrector.