In an ion source (3) in which a repeller electrode (32) for forming a repelling electric field that repels ions toward an ion emission port (311) is provided inside of an ionization chamber (31), ion focusing electrodes (36) and (37) are respectively arranged between an electron introduction port (312) and a filament (34) and between an electron discharge port (313) and a counter filament (35). An electric field formed by applying a predetermined voltage to each of the ion focusing electrodes (36) and (37) intrudes into the ionization chamber (31) through the electron introduction port (312) and the electron discharge port (313), and becomes a focusing electric field that pushes the ions in an ion optical axis C direction. Ions at positions off a central part of the ionization chamber (31) receive the combined force of the force of the repelling electric field and the force of the focusing electric field, and move toward the ion emission port (311) while approaching the ion optical axis C. Accordingly, the amount of ions sent out from the ion emission port increases. Further, even if a charge-up phenomenon occurs, the ion trajectories less easily change, and the stability of the sensitivity can be enhanced.