An electron beam apparatus is provided in which a sample is scanned with a plurality of primary electron beams respectively emitted from a plurality of electron guns. Each of the electron guns comprises a mechanism for adjusting a relative position between a cathode and anode. The adjusting mechanism of each of the electron guns comprises an insulator for supporting the cathode and a plurality of piezo elements for supporting the insulator. The piezo element varies its length in response to a voltage applied thereto. Therefore, the position of the cathode is adjustable relative to the anode position, by controlling voltages applied to the piezo elements.