This ion source generates a ribbon-like ion beam whose dimension in the Y direction is larger than the dimension in the X direction. This ion source includes a plasma generating vessel having an ion extraction port extending in the Y direction, a plurality of cathodes arranged in a plurality of stages along the Y direction on one side in the X direction in the plasma generating vessel, a reflecting electrode arranged on the other side in the X direction in the plasma generating vessel opposite to the cathodes, and electromagnets for generating magnetic fields along the X direction in regions including the plurality of cathodes in the plasma generating vessel.