Abstract: Disclosed is an apparatus for processing a substrate, the apparatus including: a housing providing a processing space therein; support unit disposed within the housing and supporting a substrate; and a plasma generating unit provided above the housing, in which the plasma supplying unit includes: a plasma chamber with a discharge space formed inside; a diffusion member provided between the plasma chamber and the housing, and diffusing plasma; a plasma source for generating plasma in the discharge space from process gas; and a sealing member provided between a lower flange of the plasma chamber and an upper flange of the diffusion member, and the sealing member includes: an inner sealing member inserted into a mounting groove formed in an upper surface of the upper flange; and an outer sealing member inserted into a space formed by combination of the upper flange and the lower flange, and positioned at a further outward side from the discharge space than the inner sealing member.