In a vacuum chamber (1), an emitter (3) and a target (7) are opposed to each other. A guard electrode (5) is disposed around an outer circumference of an electron generating portion (31) of the emitter (3). A supporting part (4) supports the emitter (3) movably in an end-to-end direction of the vacuum chamber (1). Reforming treatment is performed on the guard electrode (5) by operating the supporting part (4), moving the emitter (3) to an open end (21) side (non-discharge position) and applying a voltage to repeatedly effect discharge on the guard electrode (5) in a state where field emission from the electron generation portion (31) is suppressed. After the reforming treatment, the supporting part (4) is again operated. The emitter (3) is moved to an open end (22) side (discharge position) and placed in a state where field emission from the electron generation portion (31) is allowed.