A beam extraction slit structure includes a plasma chamber interior surface that is, in operation, in contact with a plasma; a plasma chamber exterior surface that faces an extraction electrode; and a slit surface part that forms a beam extraction slit between the plasma chamber interior surface and the plasma chamber exterior surface in the beam extraction direction. The slit surface part includes a plasma meniscus fixing part formed in an area of relatively higher plasma density in the slit longitudinal direction to fixingly maintain a plasma meniscus of the plasma and a plasma meniscus non-fixing part formed in an area of relatively lower plasma density in the slit longitudinal direction to movably maintain the plasma meniscus of the plasma in the beam extraction direction.