PURPOSE: To obtain a switching method faster in its responce time and capable of reducing the size of an ion beam optical system by allowing ion beams to be intermittently emitted corresponding to the on and off of beam-shaped light rays by intermittently irradiating the tip of an emitter chip with said light rays having a prescribed wavelength.

CONSTITUTION: Now, when with the operation of a YAG laser 10, an emitter chip 1 is irradiated with a laser beam 12, donor level electrons are excited since the emitter chp 1 is made of n type Ge, and thus the Fermi level of the emitter chip 1 rises, allowing work function tbe reduced. When a critical distance becomes large resultantly, the number of gas molcules being ionized is reduced, i.e., ion beams are effectively turned off. Then, when the operation of said YAG laser 10 is stopped to cease the irradiation of the laser beams 12, electrons in a conductive region return to the donor level by recombination, and the Fermi level also returns to the original level, so that the critical distance also returns to permit the number of molecules being ionized to be increased, allowing the ion beams to be produced again.