An insulation structure provided among a plurality of electrodes for extraction of an ion beam from a plasma generating section is provided. The insulation structure includes an insulation member including a first part connected to a first electrode and a second part connected to a second electrode and configured to support the first electrode to the second electrode, a first cover surrounding at least a part of the first part to protect the first part from contamination particles, and a second cover surrounding at least a part of the second part to protect the second part from contamination particles. At least one of the first part and the second part is made of a machinable ceramic or a porous ceramic.