An ion source for producing an ion beam utilized for fabrication and processing of semiconductors, thin films or the like includes a plasma producing chamber equipped with first magnetic means for limiting a plasma region and a plasma expansion chamber provided in combination with the plasma producing chamber on the side where a beam extracting electrode is disposed. The plasma expansion chamber is provided with second magnet array for confining and holding a plasma region therein which is of a larger area than that of the plasma region formed in the plasma producing chamber.