A vacuum container is configured so that an opening on one side and an opening on another side in the longitudinal direction of a cylindrical insulating body are sealed with an emitter unit and a target unit respectively; and a vacuum chamber is provided on the inner peripheral side of the insulating body. The emitter unit is provided with: a moving body located on the one side in the longitudinal direction in the vacuum chamber and supported so as to be movable in the longitudinal direction via a bellows; and a guard electrode located on the outer peripheral side of the moving body. An emitter section having an electron generating section is formed at a tip section of the moving body on the other side in the longitudinal direction by subjecting the surface of the tip section to film formation processing.