Disclosed is An apparatus for treating a substrate includes a chamber having a treatment space provided therein to treat the substrate and having an entrance for introducing or withdrawing the substrate, a liner disposed in the treatment space, disposed adjacent to an inner sidewall of the chamber, and having an opening formed at a position of facing the entrance to introduce or withdraw the substrate, a supporting unit to support the substrate in the treatment space, a gas supplying unit to supply process gas to the treatment space, a plasma source to produce plasma from the process gas, and a door assembly to open or close the entrance. The door assembly includes a door which includes a door unit provided outside the chamber to be movable between an opening position to open the entrance and a closing position to close the entrance, and an insertion unit extending from the door unit toward the treatment space and inserted into the opening of the liner at the closing position, and a door driving unit to drive the door.