PURPOSE: To make an electron shower quantity come into an optimum state all the time, by controlling this electron shower quantity so as to cause the sum of both electric currents, one flowing in a disc and the other in a beam target, to be turned to almost zero.

CONSTITUTION: A disc 4 and a beam target 16 both are connected to the ammeter 32 installed at the outside of a vacuum vessel 2 in parallel with each other, and a total current I3 of both currents, one is flowing in the disc 4 and the other in the beam target 16 is measured by the ammeter 32, then the current I3 is fed to a control circuit 34 which controls a filament power source 22 so as to cause the current I3 to be turned to almost zero on the basis of the current I3 to be fed out of the ammeter 32 and also controls a quantity of an electron shower 20 to be emitted out of an electron shower gun 18. With this constitution, an ion flow is neutralized by the electron shower 20 out of the electron shower gun 18, so that this state is considered to be an optimum state for control the charge of a wafer 6.