PURPOSE: To remove the distortion of an electron beam converged on a target by providing an electric field correction electrode on the opposed surface of two opposed electrodes at the inner part of an auxiliary electrode or their vicinity.

CONSTITUTION: Flat plate type electric field correction electrodes 113 and 114 are mounted on the opposed surface of a third grid 13 and a fourth grid 14. The electric field correction electrode 113 has almost the same size as a cup-shaped electrode 23b in the X-axis direction, but it has a larger diameter than the cup- shaped electrode 23b in the Y-axis direction, especially the central part 200 has a large diameter. The electric field correction electrode 114 mounted on the fourth grid 14 has also the same shape. An electron beam also becomes almost a circular beam spot on a target. In addition, three electron beams 3a, 3b, and 3c are converged on one point on the target.