A substrate processing system includes a processing container body having an opening, a lid which closes an opening, a mover for relatively moving the lid with respect to the opening to open and close the opening, and a lock mechanism which locks the lid to the processing container body. The lock mechanism includes an arm member and a locking member. The arm member is provided on one of the processing container body and the lid and extends toward the other when the processing container body is located at a position where the lid is separated from the processing container body. The locking member restricts a displacement of the arm member by being engaged with a part of the arm member. This part is located beyond the gap space when the lid is at the separated position.