PURPOSE: To improve the linearity of input and output characteristecs and productivity by forming the shape of electron beam spots into a rectangle, while keeping the electrostatic lens of an electron gun in the axially symmetrical structure.

CONSTITUTION: An electrostatic lens 6 consists of a Wehnelt cathode 9, an electrode 10, and anodes 11 to 13 and forms the axially symmetrical structure. The shape of the electron emissive area in the cathode 9 forms a rectangle. A rectangular picture of the electron emissive area can be focussed on a semiconductor detector 3 by applying a voltage to the anode 11 so that required electron current can be led by such structure, applying the beam acceleration voltage to the anode 13, and adjusting the voltage which is applied to the anode 12.