An electron beam excitation ion source includes a housing having an ion generation chamber therein, a port for supplying a discharge gas to the ion generation chamber, a porous electrode for supplying accelerated electrons to the ion generation chamber from an electron generation chamber, causing the accelerated electrons to collide against the discharge gas to generate a plasma containing ions in the ion generation chamber. The housing have an ion extraction slit port through which the ions are extracted from the ion generation chamber outside the housing. An electrode is formed around the ion extraction port, for causing a local discharge around the ion extraction port so as to guide the ions in the plasma to the ion extraction port.