A substrate processing apparatus comprises a reaction chamber which is to accommodate stacked substrates, a gas introducing portion, and a buffer chamber, wherein the gas introducing portion is provided along a stacking direction of the substrates, and introduces substrate processing gas into the buffer chamber, the buffer chamber includes a plurality of gas-supply openings provided along the stacking direction of the substrates, and the processing gas introduced from the gas introducing portion is supplied from the gas-supply openings to the reaction chamber.