Field-emission electron gun and method for controlling same

The disclosed charged particle beam apparatus includes a field-emission electron source including single crystal of tungsten; a vacuum chamber having the electron source therein; an exhausting system for exhausting the vacuum chamber; a filament connected to the electron source to let flow a current through the electron source and thereby heat the electron source; a power supply for letting a current flow through the filament; an ammeter for measuring a total current emitted from the electron source; and a controlling unit for exercising control to cause the power supply to let a current flow through the filament when the total current measured periodically has become a predetermined ratio or less as compared with a total current from the electron source found immediately after first electron beam emission, or a total current from the electron beam found immediately after a current is let flow through the filament.