A substrate processing apparatus includes: a process container configured to receive a substrate therein; a pressure detection part configured to measure an internal pressure of the process container; an exhaust-side valve installed in an exhaust pipe configured to exhaust an interior of the process container; a gas storage tank connected to the process container through a first gas supply pipe; a gas amount measuring part configured to measure an amount of gas stored in the gas storage tank; and a control valve installed in the first gas supply pipe and configured to control the internal pressure of the process container by changing an opening degree of the control valve based on the internal pressure of the process container which is detected by the pressure detection part and by controlling a flow path cross section through which the gas is supplied from the gas storage tank to the process container.