PURPOSE: To obtain a high intensity electron beam by negating a magnetic flux penetrating a negative electrode surface with a simple structure, and preventing an electron beam converged by a converging coil from being diffused when it comes out of the magnetic field.

CONSTITUTION: A heating and magnetic field negating mechanism 21 composed of a heater coil 4 to generate a line 30 of magnetic force in the opposite direction of a line 10 of magnetic force, is provided on the reverse side of a negative electrode 3 in order to negate a magnetic field composed of the line 10 of magnetic force generated by a converging coil 2 on a surface of the negative electrode 3 besides heating the negative electrode 3.