Provided is an apparatus for treating a substrate. The apparatus comprises a plasma boundary limiter unit disposed within a process chamber to surround a discharge space defined above a support unit. The plasma boundary limiter unit comprises a plurality of plates disposed along a circumference of the discharge space, and the plurality of plates are spaced apart from each other along the circumference of the discharge space so that a gas within the discharge space flows to the outside of the discharge space through passages provided between the adjacent plates.