A substrate processing apparatus includes a reaction tube defining a substrate processing chamber; a gas inlet provided at a lower portion of the reaction tube to supply a process gas; a first buffer unit for temporarily retaining the process gas, the first buffer unit at a first side of an inner surface of the reaction tube and includes a plurality of gas supply holes; and a gas outlet provided at a second side of the inner surface of the reaction tube opposite to the first side, to exhaust the process gas from the process chamber. The gas supply holes are provided from an upper end portion of the first buffer unit to a lower end portion of the first buffer unit, and the process gas is supplied through the plurality of gas supply holes into the process chamber, passes through the process chamber, and exhausted through the gas outlet.