PURPOSE: To provide a microbeam generating device which can reduce the contraction rate of a beam diameter, brings about no chromatic aberration, and can generate a microbeam with a diameter about 0.1μm

CONSTITUTION: An ion source 1, a slit 2 to compose an object point, an acceleration pipe 3 which has an inlet opening electrode 3a and an outlet opening electrode 3b, and an Einzel lens 5 which has an inlet opening electrode 5a, a central opening electrode 5b, and an outlet opening electrode 5c, are arranged in order on the axial line of ion beams. A voltage is applied between the ion source 1 and the slit 2 by the first power source 6, while a voltage is applied between the entrance of the acceleration pipe and the outlet opening electrode by the second power source 7, so as to make the central opening electrode of the Einzel lens 5 and the ion source at an equal potential. The ion beam passing through the acceleration pipe 3 is made in an divergent beam, emitted from a virtual image point at the entrance or appearance, and imaged by the Einzel lens 5 so as to form a spot 8 of the microbeam.