According to an embodiment of the present disclosure, a substrate processing apparatus including a housing is provided. The housing having an internal atmosphere of a reduced oxygen concentration includes a box structure configured to accommodate a substrate holder which receives a plurality of substrates therein and including a first gap and a second gap. Further, the housing includes an inert gas pipe connected to the box structure, and configured to supply an inert gas to the box structure, a cover member mounted in the box structure, and a buffer space formed between an internal space of the box structure and the cover member.