Abstract: An ion source for emitting an efficient radiation of ion beam having a rectangular cross section includes a set of parallel electrodes to which a microwave power is supplied to generate a microwave electric field in an electrode gap. A DC magnetic field is applied in a direction along the opposing surfaces of the electrodes to provide a microwave discharge in the electrode gap in cooperation with the microwave electric field crossing therewith. The electrode gap or discharge space has a rectangular cross section perpendicular to a direction along which ions produced by the microwave discharge are extracted as an ion beam with a side of the cross section corresponding to the distance between the electrodes being shorter than its side crossing therewith. This allows the efficient generation of the ion beam having the rectangular cross section through one or more extraction electrodes which include rectangular slits corresponding in pattern to the above-mentioned cross section.