An ion source is to extract a ribbon-shaped ion beam longer in the Y direction in the Z direction and provided with a plasma generating chamber, a plasma electrode which is disposed near the end of the plasma generating chamber in the Z direction and has an ion extracting port extending in the Y direction, a plurality of cathodes for emitting electrons into the plasma generating chamber to generate a plasma and arranged in a plurality of stages along the Y direction, and a magnetic coil which generates magnetic fields along the Z direction in a domain containing the plurality of cathodes inside the plasma generating chamber.