A substrate treating apparatus is provided which includes housing and a door assembly. The housing provides a process space for treating a substrate therein and has an opening formed at a sidewall thereof. The door assembly opens and closes the opening. The door assembly includes a shutter, a driving member, and a gap maintaining unit. The driving member transfers the shutter to an open position where the shutter faces to the opening and to a blocking position where the shutter gets out of the open position. The gap maintaining unit maintains a constant gap between the shutter and the sidewall.