The present invention provides a charged particle optical system which emits a charged particle beam, the system including an electrostatic lens, and a grid electrode opposed to the electrostatic lens along an optical axis of the electrostatic lens, and configured to form an electrostatic field in cooperation with the electrostatic lens, wherein the grid electrode is configured such that an electrode surface, opposed to the electrostatic lens, of the grid electrode has a distance, from the electrostatic lens in a direction of the optical axis, which varies with a position in the electrode surface.