PURPOSE: To obtain a signal having low noise level by providing two ionization chambers for passing a beam, a pair of high voltage capturing electrodes, and means for deriving an electric signal dependent on the beam position along a measuring axis from the capturing electrode.

CONSTITUTION: A signal indicative of the quantity of ion comprising one beam pulse derived from capturing electrodes 16, 17 is amplified through amplifiers 21, 22 and subtracted from a reference signal by means of comparators 23, 24. The reference signal is fed from a reference signal source 25, 26 has a value equal to the amplitude of a signal amplified when the beam position is correct. Current flowing through pairs of deflection coil 29, 30, 31, 32 disposed on the opposite sides of beam is controlled before an electron beam is deflected in a deflection magnet by the difference between amplified signal and reference signal passing through drive stages 27, 28. The current imparts a lateral field to the beam which is applied with a correction force normal to the direction of beam axis and field and the current is varied such that the beam is located correctly.