A neutral particle beam processing apparatus comprises a plasma generator for generating positive ions and/or negative ions in a plasma, a pair of electrodes (5, 6) involving the plasma generated by the plasma generator therebetween, and a power supply (102) for applying a voltage between the pair of electrodes (5, 6). The pair of electrodes (5, 6) accelerate the positive ions and/or the negative ions generated by the plasma generator. The positive ions and/or the negative ions are neutralized and converted into neutral particles while being drifted in the plasma between the pair of electrodes (5, 6) toward a workpiece (X). The accelerated neutral particles pass through one of the electrodes (6) and are applied to the workpiece (X).