A cylindrically-shaped auxiliary electrode and a cylindrically-shaped reflecting electrode are located anterior to a spray flow ejected from an ESI ionization probe. An inlet end of a heated capillary extends into the space between the two electrodes. The auxiliary electrode and heated capillary are grounded, while the reflecting electrode is supplied with a direct-current voltage having the same polarity as measurement target ions. As a result, a reflecting electric field which reflects ions originating from sample components and charged droplets, being carried by the spray flow, is created within the space between the two electrodes. A focusing electric field for focusing ions onto the inlet end is also created in an area near the inlet end. The ions originating from sample components are thereby separated from the gas flow and gathered around the inlet end, to be drawn into the heated capillary and sent into a vacuum chamber.