PURPOSE: To reduce the energy of ion beam, and to reduce the energy dispersion ΔE by performing the discharge inside of an ion source chamber intermittently, and taking out the beam to the outside only during the specific period, in which discharge is not performed.

CONSTITUTION: An ion source, an extraction electrode and a means, which is provided in an ion beam passage and which cuts and passes the ion beam, are provided. The discharge for exciting a material to plasma to performed intermittently, and during the excitation time (t1), in which discharge is performed, an object is not irradiated with ion beam, and only during the extraction time (t4) of a part of the stopping time (t2), in which discharge is not performed, the ion beam is passed through a beam flow passage to irradiate the object with the ion beam. At this stage, since the energy oscillation of the ion beam by the internal electric field is not generated and the electron temperature Te is lowered, the energy dispersion ΔE at an ion extraction sheath part is small. Consequently, since the fluctuation of the positive ion energy at the time of going out of the ion source chamber is small, a low-energy ion beam having a high quality, of which energy dispersion is small, is obtained.