A substrate processing apparatus includes a processing container configured to air-tightly accommodate substrates, a plurality of mounting stands configured to mount the substrates, a process gas supply part configured to supply a process gas to the mounting stands, an exhaust mechanism configured to evacuate an interior of the processing container, a partition wall configured to independently surround the mounting stands with a gap left between the partition wall and each of the mounting stands, and cylindrical inner walls configured to independently surround the mounting stands with a gap left between each of the inner walls and each of the mounting stands. Slits are formed in the inner walls. The process gas in the processing spaces is exhausted via the slits. The inner walls include partition plates for bypassing the process gas so that the process gas does not directly flow into the slits.