PURPOSE: To realize the simplification of power source structure by providing a constitution capable of varying the intensity of converging magnetic field to exciting current, and driving a converging electromagnet and a deflecting electromagnet in series.

CONSTITUTION: Magnetic poles 33-36 are constituted movably to a return yolk 32. By turning the bolt 60 of a magnetic pole driving mechanism 55, the magnetic poles 33-36 are moved to vary the intensity of the magnetic field acting on an electron beam 30. The coils 37-40 of converging electromagnets 23, 25 are connected in series to each other, and the coil of a deflecting electromagnet is connected in series thereto. An exciting current for generating a deflecting magnetic field corresponding to the energy of the electron beam is supplied. The bolt 60 is rotated in this state, thereby regulating the magnetic field to an optimum converging magnetic field.