Emitter (3) and target (7) are arranged so as to face each other in vacuum chamber (1), and guard electrode (5) is provided at outer circumferential side of electron generating portion (31) of emitter (3). Emitter (3) is supported movably in both end directions of vacuum chamber (1) by emitter supporting unit (4) having movable body (40). To perform regeneration process of guard electrode (5), emitter is moved to no-discharge position by operating emitter supporting unit, and state in which field emission of electron generating portion (31) is suppressed is set, then by applying voltage across guard electrode (5), discharge is repeated. After regeneration process, by operating emitter supporting unit again, emitter is moved to discharge position, and state in which field emission of electron generating portion (31) is possible is set with movement of movable body (40) toward the other and side being restrained by movement restraining unit (6).