According to one embodiment, a substrate processing apparatus includes: an inner tube extending in a first direction and configured to accommodate a plurality of substrates; an outer tube configured to surround the inner tube and provide an airtight sealed space; a nozzle disposed in the inner tube; a gas supply configured to supply a processing gas to the inner tube via the nozzle; at least one slit provided on a side surface of the inner tube facing the nozzle; and an exhaust port coupled to the outer tube. Along the first direction, an opening area of a central portion of the slit is larger than an opening area of end portions of the slit.