A beam processing apparatus comprises a workpiece holder (20) for holding a workpiece (X), a plasma generator for generating a plasma in a vacuum chamber (3), first electrode (4) disposed in the vacuum chamber (3), and a second electrode (5) disposed upstream of the fist electrode (4) in the vacuum chamber (3). The beam processing apparatus further comprises a voltage applying unit for applying a variable voltage between the first electrode (4) and the second electrode (5) to alternately extract positive ions (6) and negative ions from the plasma generated by the plasma generator.
