Abstract: When neutralized plasma is generated, the cylindrical electrode 8 is set at a negative potential against the processing chamber 23 by the DC power source 18, so that ions 23 in the neutralized plasma can be collected at the cylindrical electrode 8. Electrons 24 equal to the collected ion charge can be supplied uniformly toward the ion beam 25. Therefore, by allowing the cylindrical electrode to collect ions, the ion collection area can be spread easily, and only by generating neutralized plasma of low density, a sufficient volume of ions can be collected surely from the plasma and a sufficient volume of electrons can be supplied to the ion beam 25 at the same time.