An apparatus for processing a substrate includes a housing having a processing space therein, a transfer robot that loads the substrate into the processing space or unloads the substrate from the processing space, a support unit including a chuck that supports the substrate in the processing space and a lift pin that moves the substrate in an up-down direction, a dielectric plate having a lower surface disposed to face an upper surface of the chuck, and a gap measurement unit that measures a gap between the dielectric plate and the substrate supported by the lift pin or a gap between the dielectric plate and the chuck.