For absorbing a high voltage when an abnormal discharge is generated between an anode and another electrode, between an anode ADE and a control electrode G1, a surge current absorbing electrode G2 which has a plurality of electron beam passing holes AHL which allow electron beams to pass therethrough is arranged, and a DC bias power source DCG and a spark gap SG are connected in parallel between the surge current absorbing electrode G2 and a ground surface.