In a field emission electron gun including heating means for heating an anode to prevent gas emission from the anode due to bombardment thereof by the electron beam emitted from the cathode, the present invention provides a field emission electron gun including first exhaust means for defining a first chamber including the cathode and for exhausting said chamber to vacuum, and second exhaust means separate from said first exhaust means defining a chamber including said anode heating means for exhausting said chamber to vacuum independently of the first exhaust means. This construction makes it possible to maintain the chamber including the cathode constantly in the high vacuum state.