Abstract: A plasma generator generates positive ions and negative ions in a plasma. An ion extracting portion (4, 5) selectively extracts the generated positive ions and negative ions from the plasma, and accelerates the extracted ions in a predetermined direction. The positive ions and the negative ions are selectively applied to the workpiece (X). The plasma generator applies a high-frequency voltage to a process gas in a vacuum chamber for generating a plasma which is composed of positive ions and electrons from the process gas, and interrupts the high-frequency voltage for attaching the electrons to the residual process gas to generate negative ions. The application of the high-frequency voltage and the interruption of the high-frequency voltage are alternately repeated.