The charged particle beam apparatus having an opening formation member formed with an opening for passage of a charged particle beam emitted from a charged particle source, and either a detector adapted to detect charged particles having passed through the passage opening or a detector adapted to detect charged particles resulting from bombardment on another member of the charged particles having passed through the opening, comprises an aligner for aligning charged particles discharged from the sample and a control unit for controlling the aligner, wherein the control unit controls the aligner to cause it to shift trajectories of the charged particles discharged from the sample so that length measurement may be executed on the basis of detection signals before and after the alignment by the aligner.