The wafer processing apparatus includes a chamber that is pressurized to a pressure that is higher than the pressure of the exterior thereof, an opening portion through which the interior and the exterior of the chamber are in communication with each other, and a door that closes the opening portion. When the opening portion is closed by the door, a portion of the opening remains as an aperture uncovered by the door. In conventional semiconductor wafer processing apparatus, the interior of the apparatus is sealed and pressurized in order to keep a high degree of cleanness in the wafer processing portion, and therefore airflow is generated due to a pressure difference between the interior and the exterior of the apparatus. With the above feature of the invention, it is possible to suppress creation of such airflow and prevent dust from entering the wafer processing apparatus to eliminate wafer contamination.